Across:
1 Most of those at the end of 11 down (most of end of ‘dairy farmers’ = FARMER) might have growing concerns (FARMER),
4 Firm (Co) sets up (= reversal indicator) (Co = OC-) working (= anagram indicator) casino (casino = -CASION) (OCCASION) for particular event (OCCASION),
9 Like Marsh (like marsh/bog = reedy = RE-EDY) holding mike (Mike = -M-) (REMEDY) for The Cure (REMEDY),
10 Really large space (UNIVERSE) in a foreign (a = UN-) institute (-I-) near Limerick (-VERSE) (UNIVERSE),
12 Accept a challenge (UNDERTAKE) from one of those with grave responsibilities (undertaker) briefly (r) ‘undertaker’ without ‘r’ = UNDERTAKE),
13 Creatives don’t get visa (‘creatives’ don’t get ‘visa’ = crete = CRETE) for Mediterranean island (CRETE),
14 Organises (= anagram indicator) the races (the races = TEACHERS) for dogs (PETS) (TEACHER’S PETS) – the classy favourites (TEACHER’S PETS),
18 Request (ASK FOR) from 25 across (‘advice’ = ADVICE) (ASK FOR ADVICE) to seek counsel (ASK FOR ADVICE),
21 Excuse (ALIBI) to stand out of tidal basin (‘stand’ out of ‘tidal basin’ = ALIBI),
22 What do you expect when you undercook the turkeys (RARE BIRDS) for exceptional people? (exceptional people/things = RARE BIRDS),
24 Market Research ignores Esther (‘market research’ ignores ‘Esther’ = markearc = CARMAKER) Ford, for one (CARMAKER),
25 Guidance (ADVICE) rubs off on scuba diver (‘rubs’ off on ‘scuba diver’ = cadive = ADVICE),
26 Consider in a different light (REASSESS) most of (l) the actual (‘real’ without ‘l’ = REA-) engineering (-E-) found in two ships (SS-SS) (REASSESS),
27 Decorative mask (VENEER) for disguise? (VENEER).
Down:
1 Royal (-R-) in new (= anagram indicator) outfit (outfit = FO-TUIT-) contacts Yard (-Y) (FORTUITY) – is that a chance occurrence? (FORTUITY),
2 9 across isn’t finished (y) (‘remedy’ without ‘y’ = REMED-) with Ring (ring/phone = dial) losing the head (d) (‘dial’ without ‘d’ = -IAL) (REMEDIAL) with preventative action (REMEDIAL),
3 The old lead figure (ELDER) from The (el) The (der) is heard in Spain (‘the’ = EL-) and Germany? (‘the’ = -DER) (ELDER),
5 Peer (CONTEMPORARY) is awkward (contrary = CONT-RARY) around English (-E-) politician (-MP-) from Oxford (-O-) (CONTEMPORARY),
6 Stood up for (ADVOCATED) Dublin (-D) supporter (ADVOCATE-) from the start (= positional indicator) (ADVOCATED),
7 Turns up (= reversal indicator) lost (-L) listeners (ears = -SRAE-) primarily in (primarily ‘In’ = I-) (ISRAEL) middle-eastern country (ISRAEL),
8 State (= homophone indicator) did a job for the bakers (‘kneaded’ = ‘NEEDED’) when it was required (NEEDED),
11 500 (Roman 500 = D-) subject to The Draft (-AIRY= subject to the draft) could be experts in their field (FARMERS) (DAIRY FARMERS) or stockholders around Jersey? (DAIRY FARMERS),
15 Detective (-PI-) taken in by party leader (host = HOS-T-), gangsters (gangster’s = Al’s = -ALS) (HOSPITALS) and those supplying drugs (HOSPITALS),
16 Headache (MIGRAINE) for Frenchman (Monsieur = M-) travelling (= anagram indicator) Nigeria (Nigeria = -IGRAINE) (MIGRAINE),
17 Workers (MEN-) give testimony (-SWEAR) (MENSWEAR), the lads are into this type of gear (MENSWEAR),
19 One producing steps (DANCER) to remove boating in Botanic Garden (remove ‘boating’ from ‘Botanic Garden’ = carden = DANCER),
20 Redevelopment (= anagram indicator) is rare (is rare = SIERRA) in the mountain range (SIERRA),
23 Slightly move (BUDGE), for example (e.g.), one of those from the capital (Dub) going all the way back (= reverse all indicator) (eg = -GE + Dub = BUD-) (BUDGE).