Across:
1 Take issue with (OPPOSE) small operation (OP-) on model (-POSE) (OPPOSE),
4 Influenced (AFFECTED) what the player did (A-CTED) getting to grips with very loud (forte fortissimo = -FF-) final game (final ‘gamE’ = -E-) (AFFECTED).
9 Concerns of writers (THEMES) from border (-HEM-) seduced by set- (set) -back? (= reverse indicator) (set = T-ES) (THEMES),
10 Makes a meal (SANDWICH) of gritty stuff (SAND-) coming from women’s organisation (Women’s Institute = -WI-) connected to church (-CH) (SANDWICH)
12 3 down (‘swear’ = SWEAR) has information? (WORD) (SWEAR WORD) Damn! (SWEAR WORD),
13 Talk at length trivially (rabbit), for the most part (t) (‘rabbit’ without ‘t’ = RABBI), to religious teacher (RABBI),
14 Private (SECRET) Second Class (second class = the B- class) gets to give out (to give out = -ALLOT) (SECRET BALLOT) – as a politician that wouldn’t give you a vote of confidence! (SECRET BALLOT),
18 First time Julius Caesar appeared on world’s stage, for instance (first time William Shakespeare’s Julius Caesar appeared on world’s stage = OPENING NIGHT) – a window of opportunity (OPENING) and a dark period? (NIGHT) (OPENING NIGHT),
21 Ways (ROADS) out of Mediterranean Island (Rhodes) State (= homophone indicator) (‘Rhodes’ = ‘ROADS’),
22 Tore (T-ORE) around university (University of Limerick = -UL-) with the French (the = -LA-) movement heading (‘Movement’ heading = -M-) (TULLAMORE) for town in The Midlands (TULLAMORE),
24 Noticed (saw), in retrospect (= reverse indicator) (saw = WAS-), hotel (-H-) has capacity (-ROOM) (WASHROOM) for American gents (WASHROOM),
25 Upsets (= reverse indicator) Slav- (Slav = -VALS) -Irish (-I-) by writing (from the 3Rs = R-) (RIVALS) to competitors (RIVALS),
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 Harvey left heavyhearted (‘Harvey’ left ‘heavyhearted’ = heated = HEATED) and angry (HEATED).
Down:
1 The thickest (OBTUSEST) hooligan (yob) loses the head (y) (‘yob’ without ‘y’ = OB-) and takes drugs (-USES-) in race (TT race = -T-T) (OBTUSEST),
2 Play-acting (PRETENCE) in gym (P-E) over centre (centre) spread? (= anagram indicator) (centre = -RETENC-) (PRETENCE),
3 A vow (avow = SWEAR) from dowagers with no God (‘dowagers’ with no ‘God’ = waers = SWEAR),
5 Nude artfully (nude artfully) created (= anagram indicator) (nude artfully = FRAUDULENTLY) in a way that involves deception (FRAUDULENTLY),
6 Denied row (denied row) concocted (= anagram indicator) (denied row = EIDERDOWN) as cover for one of those retiring (EIDERDOWN),
7 Leaves from livery stable (‘leaves’ from ‘livery stable’ = irytbl = TRILBY) with O’Ferrall’s novel headgear (TRILBY from the heroine/Trilby O’Ferrall in George de Maurier’s Trilby),
8 Children’s writer (Roald DAHL-) introduces intelligence agent (introduces ‘Intelligence Agent’ = -IA) (DAHLIA) making an appearance in the plot (DAHLIA),
11 Search thoroughly (COMB-) island (-I-) lands (-NATIONS) (COMBINATIONS) for a number of blends (COMBINATIONS),
15 Trade show (EXPO-) for users (users) showing off (= anagram indicator) (users = -SURES) (EXPOSURES) numerous revelations (EXPOSURES),
16 Unaware (IGNORANT), Italian gent (signor) lost the head (s) (‘signor’ without ‘s ‘ = IGNOR-) with worker (-ANT) (IGNORANT),
17 Made a great deal of (STRESSED) upside-down (= reversal indicator) desserts (desserts = STRESSED),
19 Turns up (= reverse indicator) inexperienced (raw = -WAR) taking direction (East = -E-) right (-R-) at the start of primary (at the start of ‘Primary’ = P-) (PREWAR) before outbreak of hostilities (PREWAR),
20 Plant (CASSIA) with dope (-ASS-) taken in by agency (C-IA) (CASSIA),
23 It’s odd coming off side-road (‘odd’ coming off ’side-road’ = siera = ARISE) in that get-up (get up = ARISE).