I did not find this easy, taking just under 30 minutes, even though the nice long 1ac went straight in. 2 clues I only fully worked out, when writing up the proceedings, and I was rather expecting some pink squares among the green. Even the two really short ones put up a bit of fight. There’s some TLSy stuff around, balanced with sporty stuff and techy stuff, so you need rather a broad appreciation of the world of knowledge, but for the most part the wordplay is there to see you through.
I’ve marked SOLUTIONS, definitions and clues for ease of understanding
Across
1 London landmark lady matriarch constructed (9,4)
ADMIRALTY ARCH Set between the Mall and Trafalgar Square, built in honour of a lady matriarch, Queen Vic rather than by her. Once containing the grace and favour apartment enjoyed by John Prescott, now being converted into a hotel. You need the LADY MATRIARCH to provide the building blocks needed for construction of your answer.
8 Pocket: area of snooker table for pot (4)
WEED I’m pretty sure this is WEE for pocket (as in battleship, a small version thereof) plus D, on the straight edge of which the yellow brown and green are place in snooker, and from which the white is struck to (re)start the game.
9 Work of British parliament covering security breach 5,5)
BLEAK HOUSE A work by Dickens. B(ritish) HOUSE (parliament) “covers” LEAK (security breach)
10 Compromise, going by sea, returning first class (3,5)
VIA MEDIA Today’s Latin. If you go by sea, you go VIA MED(iterranean). Add AI for first class reversed.
11 Popular singer comes before poet (6)
LARKIN Philip is your poet (and librarian), constructed from IN for popular (that) singer LARK comes before. It’s suggested that the first line of his “This be the Verse” is probably the best-known in all of poetry, though not to be read aloud in the library. 13 Football body’s second tier director (10)
FASSBINDER Rainer Werner, German filmmaker, none of whose films I recall seeing (please be gentle with me). FA’S S(econd) BINDER for tier, which it is if you pronounce it tie-er. And yes, there are other film people spelt with an E.
16 Cove hidden by beach — a pity (4)
CHAP How kind of the setter to signal a hidden clue with “hidden”. BeaCH A Pity.
17 Minutes or seconds from nasty accident, steps back (4)
ACTA More Latin. I’ve just now worked out its all the second letters from nAsty aCcident, sTeps bAck
18 Reason old woman needs new park keeper (10)
GROUNDSMAN Reason is GROUNDS, old woman MA, and N(ew)
20 Did state nursing languish in misuse of drug? (6)
OPINED You are meant to be nursing PINE for languish in OD, short for overdose, misuse of drug
22 After travelling, reach old English town (8)
ROCHDALE“Travelling” suggest you’re looking for an anagram: use the letters of REACH OLD
24 Provider of instructions for golf: driver needed (10)
PROGRAMMER For: PRO, G(olf) NATO alphabet, driver: RAMMER
26 Welcome private practitioner holding various positions (4)
YOGI YO for the greeting (“yo, Blair”) and GI for the (American) private
27 Exclamation from lady in theatre exposed mother and boy, observe (3,6,4)
OUT DAMNED SPOT The Lady is Lady The Scottish Play, descending into madness as in her sleepwalking she is observed trying to wash the guilty blood from her hands. Exposed: OUT, mother: DAM, boy NED and observe: SPOT
Down
1 Ace thrilled with rap music trophy (8,3)
AMERICA’S CUP“Thrilled” is your anagram indicator, and you cull ACE, RAP and MUSIC from the clue for the fodder.
2 Precocious girl of 24’s language: that’s tasty all round! (5)
MADAM Any of us who have had daughters will know that, love them as we may, they can sometimes be right little madams, which designation fits the bill. Know (if you’re Jim) or guess that ADA is a programming language (see 24). Named for Ada Lovelace, assistant to the amazing Charles Babbage, probably recognising better than he did what his marvellous mechanical computer could do, not without reason described as the world’s first computer programmer. MM for “that’s tasty” embraces ADA. 3 Marketing ploy of a number that’s taken in wrong spirit (9)
REBADGING REG(istration plate) for a number has taken in BAD GIN for wrong spirit
4 Coming to blows with general over charge (7)
LEEWARD In sailing (I hope I’ve got this right) leeward is sailing in the direction in which the wind is blowing. If you squint a bit, coming to blows translates that. LEE is your general (if nothing else, think Dukes of Hazard) and charge supplies WARD, someone you are responsible for.
5 Rustic couple on lake (5)
YOKEL Couple provides YOKE, either as in a pair of (eg) oxen or the means by which they are joined. Add L(ake)
6 They’re ok for cars, for transporting (4,5)
ROOF RACKS A decent &lit: transporting the anagram indicator and OK FOR CARS the fodder. I have always believed Bob Marley had one over his head (Is This Love?) perhaps in some VW camper van, but it appears not.
7 Bears in the wood with horse coming up (3)
HAS The wood is ASH, and the H(orse) is promoted to the top.
12 I seize promotion without difficulty: that’s viewed unfavourably (2,1,3,5)
IN A BAD LIGHT I NAB (seize) AD (publicity) LIGHT (without difficulty)
14 Mark coming soon to open street party (4,5)
STAG NIGHT One of many meanings of TAG is mark. Coming soon gives you NIGH. Place both (they’re to open) in ST(reet)
15 Single minded about success, ultimately display timidity (3,6)
RUN SCARED Another I’ve only just worked out. Single is RUN (today’s cricket). Minded gives CARED, and include succesS ultimately. Terrific misdirection.
19 Four, maybe, twisting one’s arm (7)
OARSMEN Twisting the letters of ONE’S ARM for a coxless four.
21 Girl in papers upset by gossip (5)
DIANA So apposite is could be from the Daily Express. Papers are ID, to be “upset”, and gossip give ANA. From Chambers: “a collection of someone's table talk or of gossip, literary anecdotes or possessions.”
23 Stopwork in kitchen (3,2)
DRY UP A double definition. Drying up is what we used to do with tea towels before the advent of dishwashers.
25 Character of Greek cowherd turning up off and on (3)
RHO Reverse alternate letters in cOwHeRd.