Across:
1 Austrian composer (Gustav MAHLER) playing (= anagram indicator) Harlem (Harlem = MAHLER),
4 Make it very clear (SPELL OUT) flying (= anagram indicator) pollutes (pollutes = SPELL OUT),
9 Straight man loses sight (‘straight man’ loses ‘sight’ = traman = MANTRA) of what gets repeated (MANTRA),
10 Mimic (SIMULATE) is (is) upset (= reverse indicator) (is = SI-) by strange (= anagram indicator) amulet (amulet = -MULATE) (SIMULATE),
12 Sharp-eyed type (OBSERVANT) bravest on (bravest on) manoeuvres (= anagram indicator) (bravest on = OBSERVANT),
13 Unfair to leave fundraisers (‘unfair’ to leave ‘fundraisers’ = drses = DRESS) in Mini (mini DRESS),
14 Doesn’t surrender (HOLDS THE FORT) what one does when the boss is away (HOLDS THE FORT),
18 Made oneself known again (REINTRODUCED) to shady type (red = RE-D) covering launch (-INTRODUCE-) (REINTRODUCED),
21 Business people (SUITS) closing 11 down (closing ‘bathing suits’ = SUITS),
22 Stage (STEP-) actress at finale (‘actresS’ at finale = -S) in Northern Ireland (DOWN) (STEPS DOWN) retires (STEPS DOWN),
24 Two pubs (bar + bar = BARBAR-) for one (-I-) starting college (starting ‘College’ = -C) (BARBARIC) – it’s savage! (BARBARIC),
25 10K (-RUN-) in Belgium (B-E-) won (won) reportedly (= homophone indicator) (‘won’ = ‘one’ = -I) (BRUNEI) by Malaysian neighbour (BRUNEI),
26 Judge again (REASSESS) is about (RE-) to take a (-A-) boat (-SS-) from Spain (-E-) around seas (around ‘SeaS’ = -SS) (REASSESS),
27 Top (top/jumper = JERSEY) player tries it on (JERSEY).
Down:
1 A (-A-) couple of Frenchmen (-MM-) taken in by local Irish girlfriends (from the Irish, linked to “maith an cailin” = “moths/girlfriends” = M-OTHS) (MAMMOTHS) – the beastly sorts who have long since gone (MAMMOTHS),
2 Working (= anagram indicator) on the sly (on the sly = HONESTLY) in good faith? (HONESTLY),
3 Upsetting (= reverse indicator) extract in The Mirror regurgitated (extract in ‘the mirROR REgurgitated’ = rorre = ERROR) by mistake (ERROR),
5 Spike (POINT) and Oscar (O-) Furlong (-F) heard this from the judge (ORDER) (POINT OF ORDER) querying what’s said in The House (POINT OF ORDER),
6 Cleaned up and ironed out (LAUNDERED) three articles from three countries (la + une + der = LAUNDER-) linked to writer (editor = -ED) (LAUNDERED),
7 Gives one’s address (ORATES), content to offset a roadblock (content to ‘offSET A ROadblock’ = setaro) going all the way back (= reversal indicator) (setaro = ORATES),
8 Takes cues from sheet music (takes ‘cues’ from ‘sheet music’ = hetmsi = THEISM) and belief in the creator (THEISM),
11 Graduate (BA-) gets to object (object/item = -THING) to legal actions (SUITS) (BATHING SUITS) by a number of outfits hiding beach bums presumably (BATHING SUITS),
15 Gets the picture (SEES) for endless (end ‘lesS’ = S-) sailors on old boat (-TARS) (SEES STARS) with reels (SEES STARS),
16 Reports (ACCOUNTS) cons cut a (cons cut a) settlement (= anagram indicator) (cons cut a = ACCOUNTS),
17 Put a name to (IDENTIFY) model (= anagram indicator) fined (fined = IDEN-F-) over backcombing (= reversal indicator) it (it = -TI) onto ponytail? (‘ponY’ tail = -Y) (IDENTIFY),
19 Lads falling off sailboards (‘lads’ falling off ‘sailboards’ = siboar = ISOBAR) – it could be the pressure staying on a line (ISOBAR),
20 The range (SIERRA = Spanish mountain range) of some folksier rapper (some ‘folkSIER RApper = SIERRA),
23 Try to persuade (-URGE) after (= position indicator) school (S-) (SURGE) in Rush (rush/gush = SURGE).