Across:
1 Assumes an upright position (STANDS) in ship (SS = S-S) going around Trevose Head (‘Trevose’ head = -T-) in addition (AND) (STANDS)
4 Incidents (EPISODES) of foreign (= anagram indicator) spies (spies = EPIS-S) hiding most of (c) the code (‘code’ without ‘c’ = -ODE) (EPISODES),
9 Once a year (ANNUAL) idealists leave Aleutian Islands (‘idealists’ leave ‘Aleutian Islands’ = uanlan = ANNUAL),
10 Couple of beastly figures (ass + ass = ASSASS-) up (= reversal indicator) north (NI = -IN) (ASSASSIN) capable of getting away with murder (ASSASSIN),
12 Segregate (segregate) bubbly (= anagram indicator) (segregate = EASTER EGG) from a seasonal gift (EASTER EGG),
13 Bring to bear without Briton (‘bring to bear’ without ‘Briton’ = gbear = BARGE) is lighter? (BARGE),
14 A French- (‘a’ French = UN-) -American (-A-) cold (-C-) mushroom (-CEP-) meal (-TABLE) (UNACCEPTABLE) is not satisfactory (UNACCEPTABLE),
18 Don’t show for date (STAND-UP) going with 20 down (‘comics’) shortly (s) (‘comics’ without ‘s’ = COMIC) (STAND-UP COMIC) to The Joker (STAND-UP COMIC),
21 Before The Net (before the net is the GROSS) of course? (course) That’s what I heard! (= homophone indicator) (‘course’ = ‘coarse’ = GROSS),
22 Radical (EXTREMIST) unholy (= anagram indicator) Reims text (Reims text = EXTREMIST),
24 Spill the beans on whopper (NAIL A LIE) for Spike (NAIL), Ali (Ali = A LI-) and Earl (-E) (NAIL A LIE),
25 Digs up (= reverse indicator) metal (tin = -NIT-) found in, for example (e.g. = -G-E), iodine (I-) (IGNITE) from fire (IGNITE),
26 Gets mare (gets mare) moving (= anagram indicator) (gets mare = GAS METER) with device measuring the running when the heat is on (GAS METER),
27 Unfinished (t) shelter (‘tent’ without ‘t’ = TEN-) causes offence to religious type (sin) up (= reversal indicator) (sin = -NIS) (TENNIS) for the game (TENNIS).
Down:
1 Director’s shoot (-TAKE) in old French capital (old French capital/money = sou = S- -OU-) – the first (‘The’ first = -T) (STAKE-OUT) that requires detectives to work in secret (STAKE-OUT),
2 One can’t remember (AMNESIAC) academicians dropping acid (‘academicians’ dropping ‘acid’ = aemcians = AMNESIAC).
3 Duck (DRAKE), bird, snake ignored bins? (‘bird snake’ ignored ‘bins’ = rdake = DRAKE),
5 Tweet (POST-) by bachelor (-GRADUATE) (POSTGRADUATE) working in research facility presumably qualified by the Department of Education? (POSTGRADUATE),
6 Disorganised (SHAMBOLIC) cook (= anagram indicator) boils ham (boils ham = SHAMBOLI-) with head of cauliflower (head of ‘Cauliflower’ = -C) (SHAMBOLIC),
7 Turns one’s back on (DESERT) piano out of desperation (‘piano’ out of ‘desperation’ = desert = DESERT),
8 One doing the numbers (SINGER) for OAP from Singapore (‘OAP’ from ‘Singapore’ = singre = SINGER),
11 In waiting for answer, presumably someone clocked this (RESPONSE TIME) might be how long it took the congregation to reply to priest (RESPONSE TIME),
15 It (-IT-) is found by bottom of (bottom of ‘bY’ = -Y) street (ST-) on a (-A-) square (-T- square) in Clare (CE = C- -E) (CITY STATE) and, in the past, Athens (CITY STATE),
16 Morsel (-BIT-) swallowed by Naomi (Naomi) – model (= anagram indicator) (Naomi = AM-ION) (AMBITION) with a sense of purpose (AMBITION),
17 Spreads bits about (SCATTERS) rock (= anagram indicator) stars, etc. (stars etc = SCATTERS),
19 Punch (EGGNOG) knocks over (= reverse indicator) great (-G) individual (one = -E-NO-) around middle of fight (middle of ‘fiGht’ = -G-) for good (G -) (EGGNOG),
20 No liberals in social climber’s (no ‘liberals’ in ‘social climbers’ = ocicms = COMICS) childish magazines (COMICS),
23 Covered up (= reverse indicator) by Helga Estby (covered up by ‘hELGA Estby’ = elgae = EAGLE) – a sharp-eyed high-flyer (EAGLE).