Across:
1 Rejects (= reversal indicator) painting (art = TRA-) by university (-U-) graduate (-MA) (TRAUMA) – it comes as a shock (TRAUMA),
4 It’s a stroke of luck (AS A BONUS) for a (A-) school (-S) head, missing (g) in West African country (“Gabon” without “g” = A BON-), contacting my group (-US) (AS A BONUS),
9 Woolly type (SHAGGY) of horse (-GG-) held by Seamus (SHA-Y) (SHAGGY),
10 Arrogant (arrogant = TARRAGON) cook (= anagram indicator) found in herb garden (TARRAGON),
12 Type of stick (TRUNCHEON) one gives to policeman (TRUNCHEON),
13 Some complex transformation (some “complex transformation” = EXTRA) similar to 4 across (EXTRA is similar “as a bonus”),
14 Local money (BREAD) for foodie type of junket (junket is a milk-based PUDDING) (BREAD PUDDING) might get a mention in the sweet talk? (BREAD PUDDING),
18 Won’t (wont = TOWN) run (= anagram indicator) working out (PLANNING) (TOWN PLANNING) – that’s the type of exercise that appeals to city architects (TOWN PLANNING),
21 Finishes off both (k + m) catch (“hook” without “k” = HOO-) and type of pork (“ham” without “m” = - HA) (HOO-HA) with little fuss (HOO-HA),
22 Slurring (INSULTING) from cheap shot (INSULT-) with gin (gin = -ING) cocktail (= anagram indicator) (INSULTING),
24 Parking (-P) for staff (STICK-) in America (US = U- -S) (UP STICKS) for shift (UP STICKS),
25 All together (IN TOTO) in (IN) ’22 (“two” + “two” = “to + to” = TOTO) hearing (= homophone indicator) (IN TOTO),
26 Tiny cast (tiny cast = SANCTITY) produced (= anagram indicator) a show of godliness (SANCTITY),
27 Reinforce (BEEF UP) muscle (BEEF) and tops of upper pecs (tops of “upper pecs” = UP) (BEEF UP).
Down:
1 Try out (TEST) pipe (TUBE) (TEST TUBE) for experimental purposes (TEST TUBE),
2 A (A-) retiring hack in New York (in US, a hack = cab = -BAC-) takes drugs (-USES) (ABACUSES) with calculating types (ABACUSES),
3 Sleight of hand (MAGIC) from wise guys (MAGI-) and leading criminal (leading “criminal” = -C) (MAGIC),
5 An illuminating feature of The Abbey (STAINED GLASS) is not popular in the pub (stained/dirty glass = STAINED GLASS),
6 Surprise (BARGE IN ON) on the canal (BARGE) that’s popular (IN) with family member (son) heading off (s) (“son” without “s” = ON) (BARGE IN ON),
7 Deny (NEGATE) special (= anagram indicator) agent (agent = NEGAT-) is going to Spain (-E) (NEGATE),
8 Grammatical structure (SYNTAX) of nasty (nasty = SYNTA-) novel (= anagram indicator) with mystery man (-X) (SYNTAX),
11 Payback (payback = RETURN) for going over the speed limit (TICKET) (RETURN TICKET) – it’s because of this you’ll be back where you started (RETURN TICKET),
15 To give you a clue (DROP A HINT) – most of (w) the word (“word” without “w” = ord = DRO-) coming back (= reversal indicator) is to decorate (paint = -P A -INT) around hotel (H-) (DROP A HINT),
16 Make friends (HIT IT OFF) with crowd-pleaser (HIT) and one (I-) posh bloke (toff = -T OFF) (HIT IT OFF),
17 Seems like a couple of years since (AGE) The Band (GROUP) (AGE GROUP) included those from the 70s (AGE GROUP),
19 Rush (rush = SHRU-) off (= anagram indicator) for empty (empty/remove the middle = “u”) bus (“bus” without “u” = -BS) (SHRUBS) with Maple and Ivy (SHRUBS),
20 Ease up on (LOOSEN) salad (= anagram indicator) on sole (on sole = LOOSEN),
23 Spring (LUNGE) left (L-) organisation (United Nations = -UN-) for Rising (= reversal indicator), for example (e.g. = -GE) (LUNGE).