Maybe everyone else will find it a breeze, but I would say that this is definitely not one for the Quickie brigade.
ACROSS
1 Good girl we're told deciding it's an unfair obstacle (5,7)
GLASS CEILING - G LASS sounds like 'sealing'
9 Greek festival inspires king – it’s something in the atmosphere (5)
ARGON - R in AGON; a Greek festival, such as the one at Olympia, at which competitors vied for laurel wreaths et al
10 Musical style it's rude to sing (9)
BLUEGRASS - BLUE GRASS (rat on someone)
11 The cloth sample on skimpy garments (8)
MINISTRY - TRY on MINIS
12 Leave the neighbouring housing compound (6)
ETHENE - hidden in[leav[E THE NE[eighbouring]; I'd herard of ETHANE but not this stuff
13 Investigator on the booze (8)
REPORTER - RE PORTER (a dark sweet ale brewed from black malt - one of my favourite tipples)
15 Heavy drinks bottles heading for tavern (6)
ROTUND - T[avern] in ROUND ('drinks bottles heading for tavern' is crosswordese for chuck the initial letter of tavern in drinks)
17 Reduce by cooking in two months (6)
DECOCT - DEC OCT; if I was being really honest (something I haven't tried for some time now) I would have to say that I'd never heard of this word. I probably said that last time it came up...
18 Flatter, or fairly flat? (8)
BLANDISH - if something were not that dull, but definitely on the dull side (think Michael MacIntyre doing yet another observational domestic sketch), then you might call it on the bland side, or indeed, 'blandish'. Oh God, I think I've just given him an idea he'll be able to pad out into a 3-minute item...
20 Prickly Scandinavian hero content to leave Norway (6)
THORNY - THOR N[orwa]Y
21 Sordid party game (8)
BASEBALL - BASE BALL
24 View short camera shot, in which a lot is crooked? (5,4)
CRIME WAVE - anagram* of VIEW CAMER[a]
25 Fiddle one misplaced? There it is! (5)
VOILA - VOILA with the I moving about
26 Christian book favouring giving up almost everything (12)
PROCESSIONAL - PRO CESSION (giving up) AL[l]; a book as well as 'Once in Royal David's City' then
DOWN
1 What may qualify school to provide set of rules (7)
GRAMMAR - one of the words that may premodify school, along with finishing and special needs, is GRAMMAR
2 Front of chest in great pain, so could be this? (6,8)
ANGINA PECTORIS - C[hest] IN GREAT PAIN SO* for the heart-related chest pain that my healthy lifestyle has so far kept me safe from
3 Passage south across the Atlantic (5)
SINUS - S IN US
4 Got on vessel in eastern sea (8)
EMBARKED - BARK in E MED
5 Flamboyant old Law Lord entertains (4)
LOUD - O in LUD; it is a sad indictment of modern times that judges are no longer addressed by barristers with their hands pressed against their chests under the lapels of their gowns as M'Lud. Boring old Milord is now the order of the day in the lego-judicial industry.
6 New wig and thong, kinky bedroom gear (9)
NIGHTGOWN - n[ew] WIG THONG*; shame on anyone who went down the leather, wrist-cuff route...
7 Invalid with a nature that's sickly? (14)
VALETUDINARIAN - INVALID A NATURE* for the hypochondriac
8 Get promoted in a post broadcasting (6)
ASCEND - A sounds like 'send'
14 Trained to run race, his performance is telling (9)
RACONTEUR - TO RUN RACE*
16 They could hack off Conservative Brexiteers (8)
CLEAVERS - C LEAVERS (bless 'em - I'd never have joined in the first place)
17 Didn't like touring Croatia's capital, back in Split (6)
DETACH - HATED reversed around C[roatia]
19 Item for traveller knocking on mansion walls (7)
HOLDALL - OLD (knocking on - geddit?) in HALL; walls is the containment indicator
22 Brief verse in French mostly useless (5)
ENVOI - EN VOI[d]; a literary entity that is still commonly found in crosswords - along with EPODES, ELEGIES et al - meaning a short stanza concluding a ballade
23 Demonstrate putting away hot alcoholic drink (4)
MARC - MARC[h]