This started easily with the two geisha clues, but got progressively harder. 14ac, 17ac and 7dn were all difficult. My choice for the clue of the day is 20ac – nice definition and nice wordplay. Overall I rated it a bit harder than average. Thanks to the setter.
Clues are reproduced in blue, with the definition underlined. Anagram indicators are bolded and italicised. The answer is IN BOLD CAPS, followed by the wordplay. (ABC)* means 'anagram of ABC', {deletions are in curly brackets}.
Across
1 Improv performance's block seating? (3,7)
JAM SESSION: JAM=block / SESSION = seating (at a restaurant, perhaps).
7 Choice of temperature scale in two-way split (4)
FORK: F=Fahrenheit / OR / K=Kelvin. I was expecting the choice to be Fahrenheit or Centigrade, but FORC doesn’t work! The Kelvin scale uses the same units as Centigrade, but starts at a temperature of “absolute zero”, (−273.15°C or −459.67°F, if you must ask), rather than the freezing point of water.
9 Let vitality go, becoming motionless (8)
FREEZING: FREE / ZING.
10 Expansion of g (gee) giving record award (6)
GRAMMY: GRAM=g / MY=gee.
11 Middle of cedar tree to give patchy shade (6)
DAPPLE: {ce}D{ar} / APPLE.
13 Thin computer disk lay rejected in corner (8)
NOTEBOOK: O = disk / BET=lay, all reversed and put inside NOOK=corner.
14 Producer of alcohol surprisingly not sold as miniatures? (12)
MICROBREWERY: cryptic definition, I suppose. Not a class of clue I love.
17 Change ideas of Tintin and co, read anew (12)
INDOCTRINATE: (TINTIN CO READ*).
20 Plot turned on heading off kindly English (8)
ENGINEER: RE=on / {b}ENIGN=kindly / E=English, all reversed.
21 Line cutting cube that produces smoky effect (3,3)
DRY ICE: RY=railway, in DICE.
22 Moan about Liberal receiving admiration from the East (6)
BEWAIL: AWE in LIB, all reversed.
23 First Mother Mary spoiled name for ordinary person (8)
EVERYMAN: EVE / (MARY*) / N.
25 Some honour geisha's desire (4)
URGE: hidden.
26 I'm no fun, as a result of sweaty night? (3,7)
WET BLANKET: self-evident.
Down
2 A twit not finishing a twitch, making some waves? (8)
ADRIATIC: A / DRI{p} / A / TIC. Waves are not the first thing I think of, but no doubt on a windy day …
3 Girl's fat reduced by 25 per cent (3)
SUE: SUE{t}.
4 Coarse people from Victoria and Newcastle areas? (5)
SWINE: the postcode for Victoria (the one in London, obviously), and the compass direction for Newcastle (the one in England, again obviously).
5 Frank bringing home forward but artless girl (7)
INGENUE: GENUINE, with IN=home moved up.
6 Fashion range with clothes for after dark (9)
NIGHTWEAR: (RANGE WITH*).
7 Following beat, cheerfulness short of uniform exuberance (11)
FLAMBOYANCY: F=following / LAM=beat / B{U}OYANCY.
8 Memory stick, one without flexibility (6)
RAMROD: RAM = memory / ROD = stick.
12 To share annoyance about lorry was annoying (11)
PARTICIPATE: PIP=annoyance around ARTIC=lorry / ATE=was annoying.
15 Temporary ally departs further education amid outcry (9)
BEDFELLOW: D=departs / FE=further education, all in BELLOW=outcry. Chambers defines this as just “ally”, but Collins has the sense of “temporary”.
16 Farm animals forced to abandon leader, penned here? (8)
STOCKADE=farm animals / {m}ADE = forced.
18 Cold, alien temperature round planet's mid-sized mountain (7)
CORBETT: C=cold / ET=the setter’s film of first choice, around ORB=planet. In Scotland apparently, a Munro is a mountain with a height over 3,000 feet (914 m), a Corbett is 2,500 to 3,000 ft (762–914 m) and a Graham is 2,000 to 2,500 ft (610–762 m).
19 Make desired halt at attention (6)
ENDEAR: END / EAR.
21 Live and prosper, lacking nothing (5)
DWELL: D{o} WELL.
24 Some pay for geisha's desire (3)
YEN: either desire, or the currency in geisha land.
Clues are reproduced in blue, with the definition underlined. Anagram indicators are bolded and italicised. The answer is IN BOLD CAPS, followed by the wordplay. (ABC)* means 'anagram of ABC', {deletions are in curly brackets}.
Across
1 Improv performance's block seating? (3,7)
JAM SESSION: JAM=block / SESSION = seating (at a restaurant, perhaps).
7 Choice of temperature scale in two-way split (4)
FORK: F=Fahrenheit / OR / K=Kelvin. I was expecting the choice to be Fahrenheit or Centigrade, but FORC doesn’t work! The Kelvin scale uses the same units as Centigrade, but starts at a temperature of “absolute zero”, (−273.15°C or −459.67°F, if you must ask), rather than the freezing point of water.
9 Let vitality go, becoming motionless (8)
FREEZING: FREE / ZING.
10 Expansion of g (gee) giving record award (6)
GRAMMY: GRAM=g / MY=gee.
11 Middle of cedar tree to give patchy shade (6)
DAPPLE: {ce}D{ar} / APPLE.
13 Thin computer disk lay rejected in corner (8)
NOTEBOOK: O = disk / BET=lay, all reversed and put inside NOOK=corner.
14 Producer of alcohol surprisingly not sold as miniatures? (12)
MICROBREWERY: cryptic definition, I suppose. Not a class of clue I love.
17 Change ideas of Tintin and co, read anew (12)
INDOCTRINATE: (TINTIN CO READ*).
20 Plot turned on heading off kindly English (8)
ENGINEER: RE=on / {b}ENIGN=kindly / E=English, all reversed.
21 Line cutting cube that produces smoky effect (3,3)
DRY ICE: RY=railway, in DICE.
22 Moan about Liberal receiving admiration from the East (6)
BEWAIL: AWE in LIB, all reversed.
23 First Mother Mary spoiled name for ordinary person (8)
EVERYMAN: EVE / (MARY*) / N.
25 Some honour geisha's desire (4)
URGE: hidden.
26 I'm no fun, as a result of sweaty night? (3,7)
WET BLANKET: self-evident.
Down
2 A twit not finishing a twitch, making some waves? (8)
ADRIATIC: A / DRI{p} / A / TIC. Waves are not the first thing I think of, but no doubt on a windy day …
3 Girl's fat reduced by 25 per cent (3)
SUE: SUE{t}.
4 Coarse people from Victoria and Newcastle areas? (5)
SWINE: the postcode for Victoria (the one in London, obviously), and the compass direction for Newcastle (the one in England, again obviously).
5 Frank bringing home forward but artless girl (7)
INGENUE: GENUINE, with IN=home moved up.
6 Fashion range with clothes for after dark (9)
NIGHTWEAR: (RANGE WITH*).
7 Following beat, cheerfulness short of uniform exuberance (11)
FLAMBOYANCY: F=following / LAM=beat / B{U}OYANCY.
8 Memory stick, one without flexibility (6)
RAMROD: RAM = memory / ROD = stick.
12 To share annoyance about lorry was annoying (11)
PARTICIPATE: PIP=annoyance around ARTIC=lorry / ATE=was annoying.
15 Temporary ally departs further education amid outcry (9)
BEDFELLOW: D=departs / FE=further education, all in BELLOW=outcry. Chambers defines this as just “ally”, but Collins has the sense of “temporary”.
16 Farm animals forced to abandon leader, penned here? (8)
STOCKADE=farm animals / {m}ADE = forced.
18 Cold, alien temperature round planet's mid-sized mountain (7)
CORBETT: C=cold / ET=the setter’s film of first choice, around ORB=planet. In Scotland apparently, a Munro is a mountain with a height over 3,000 feet (914 m), a Corbett is 2,500 to 3,000 ft (762–914 m) and a Graham is 2,000 to 2,500 ft (610–762 m).
19 Make desired halt at attention (6)
ENDEAR: END / EAR.
21 Live and prosper, lacking nothing (5)
DWELL: D{o} WELL.
24 Some pay for geisha's desire (3)
YEN: either desire, or the currency in geisha land.