Across:
1 Could start (‘could’ start = C-) sit-in (demo = -OMED-) passing around (= reversal indicator) unknown quantity (-Y) (COMEDY) of the funny stuff (COMEDY),
4 They don’t believe (SCEPTICS) cynic (SCEPTIC-) at school (-S) (SCEPTICS),
9 No tin in essential (no ‘tin’ in ‘essential’ = esseal = EASELS) supports for painters (EASELS),
10 Ration out (ALLOCATE) cook’s (cooks = anagram indicator) local tea (local tea = ALLOCATE),
12 Jobs for people like Polly (“Polly put the kettle on, we’ll all have tea” = jobs for people like Polly = TEA MAKERS) and Earl Grey (TEA) from The Producers (MAKERS) (TEA MAKERS),
13 ISIS airbrushed out of sensitive (‘ISIS’ airbrushed out of ‘sensitive’ = entve = EVENT) incident (EVENT),
14 System taking credit (HIRE PURCHASE) for charter (HIRE), by (“by” = “buy” = PURCHASE) the sound of it (= homophone indicator) (HIRE PURCHASE),
18 One of those government types (DICTATOR-) with small (-S-) cool (-HIP) (DICTATORSHIP) way of keeping the people under the thumb (DICTATORSHIP),
21 Tailor (ADAPT) with a (A-) type of accommodation (pad = -DAP-) up (= reversal indicator) on square (-T square) (ADAPT),
22 Sowed the seeds of (GENERATED) what’s passed on by ancestors (GENE-) and was considered special (-RATED) (GENERATED),
24 Drag into court (SUBPOENA) from A-One pubs (A one pubs = SUBPOENA) drunk (= anagram indicator),
25 Break the news to (INFORM) popular (IN-) model (-FORM) (INFORM),
26 A mistral (a mistral = ALARMIST) playing (= anagram indicator) with Cassandra (ALARMIST),
27 Clown (comic = CO-MIC) admits initial sketch (initial ‘sketch’ = -S-) (COSMIC) is out of this world (COSMIC).
Down:
1 The E-Cash (The E-Cash = CHEETAHS) forged (= anagram indicator) by wild fast types (CHEETAHS),
2 Moving (= anagram indicator) camera (camera = MA-ACRE) captures old police force (-SS-) (MASSACRE) in Night of the Long Knives (Hitler’s June 1934 MASSACRE of the Brownshirts),
3 Bottle of Zinfandel taken (bottle of ‘Zinfandel taken’ = DELTA) by the mouth (DELTA),
5 What The Boss does (CALLS THE TUNE) with requests (CALLS) for Born in The USA (THE TUNE) (CALLS THE TUNE),
6 Make out that (PROFESS-) old (-O-) Republican (-R) (PROFESSOR) is teaching others a lesson (PROFESSOR),
7 Everything is backed up (= reverse all indicator) by leader’s generous attention (leaders ‘generous attention’ = ga = -AG-) welcomed by The House (semi = IM-ES) (IMAGES) Representatives (IMAGES),
8 Is (is = SI-) setting up (= reversal indicator) train (= anagram indicator) set (set = -EST-) from midday (mid ‘day’ = -A) (SIESTA) for the rest of the afternoon in Madrid (SIESTA),
11 Spies (SECRET AGENTS) one of The Classifieds (SECRET) for American (A-) males (-GENTS) (SECRET AGENTS),
15 Add colour to (PAINT) the hall (ROOM) (PAINT ROOM) where artist keeps his coats? (coats of paints / PAINT ROOM),
16 Cold (C-) nightcap, for instance (-HAT), from kitchen (ROOM) (CHAT ROOM) is a talking point (CHAT ROOM),
17 A sudden undesirable phenomenon (EPIDEMIC) produced by barbaric (= anagram indicator) Medici (Medici = -IDEMIC) after (= positional indicator) revolting (= reversal indicator) exercise (PE = EP-) (EPIDEMIC),
19 Plant (CASSAIA) with dope (-ASS-) taken in by spies (C-IA) (CASSIA),
20 Cultural organisation (Gaelic Athletic Association = GAA = GA-A) welcomes leaders of musical band in (leaders of ‘musical band in’ = MBI-) (GAMBIA) part of Africa (GAMBIA),
23 Excited (= anagram indicator) donor (donor = RONDO) repeats refrain in concert hall (RONDO).