Across:
1 Mouthy type (G-UM) admits turning (= reversal indicator) secret agent (spy = -YPS-) (GYPSUM) involved in the construction of The Wall (GYPSUM),
4 Shock (ASTONISH) as Martin’s travelling companion (ASTON- Martin car) is (-IS-) going to hospital (-H) (ASTONISH),
9 Snow from townscapes (‘snow’ from ‘townscapes’ = tcapes = ASPECT) and strand (ASPECT),
10 Tense (tense) play (= anagram indicator) (tense = -SENTE-) taken in by Lincoln (Abe Lincoln = AB-E) (ABSENTEE) – The Man Who Wasn’t There? (ABSENTEE),
12 One of those in stable environment (HORSE-) going with loafer? (-SHOE) (HORSESHOE) Good luck with that! (HORSESHOE),
13 Travels everywhere return on one day (SANTA) with dancer’s boss (SANTA is reindeer Dancer’s boss),
14 When Henry VIII broke with The Pope perhaps (CHURCH WAR-), this study (-DEN) (CHURCHWARDEN) once kept the order in order? (CHURCHWARDEN),
18 A bench perhaps (RESTING PLACE) in the graveyard (RESTING PLACE),
21 Break loose avoiding laser (‘break loose’ avoiding/without ‘laser’ = bekoo = E-BOOK) – the one you can buy online (E-BOOK),
22 Is very realistic (PRAGMATIC) in playing (= anagram indicator) magic part (magic part = PRAGMATIC),
24 Has a hunch (SUPPOSES) drink (SUP-) is for models (-POSES) (SUPPOSES),
25 Foreign dish (PAELLA) in spring (leap) served up (= reverse indicator) (leap = PAEL-) with slaw (slaw), excluding the sides (‘s’ + ‘w’) (‘sLAw’ without ‘s’ + ‘w’= -LA) (PAELLA),
26 Nurse (TREAT-) is (-IS-) from Spain (-E) (TREATISE) in the writing study (TREATISE),
27 Support, in a manner of speaking (ASSENT), for sanest (sanest) boss (= anagram indicator) (sanest = ASSENT).
Down:
1 The Irish love (Irish ‘love’ = grá = GRA-) hothead (‘Hot’ head = -H-) in photos (-P-ICS) (GRAPHICS) and artwork (GRAPHICS),
2 Father (POP) has Mass (mass/crowd = GROUP) (POP GROUP) with band of musicians (POP GROUP),
3 Part of the family (UNCLE) from a foreign (a = UN-) line (-L-) in Clare (-C-E) (UNCLE),
5 Train (= anagram indicator) near Eastburn (near Eastburn = SUBTERRANEAN) in The Underground (SUBTERRANEAN),
6 Out at night, more so than usual (OVERSLEEP) – it’s what she does not having met handsome prince yet (Sleeping Beauty OVERSLEEP),
7 Keen (INTENT) or slow to leave Tinseltown? (‘slow’ to leave ‘Tinseltown’ = tinetn = INTENT),
8 Weather ship dismisses trips (‘weather ship’ dismisses ‘trips’ = weaheh = HEE-HAW) to Bray? (bray/donkey cry = HEE-HAW),
11 Those playing their part by going on strike? Presumably (show stoppers = SHOWSTOPPERS), they attract prolonged applause! (SHOWSTOPPERS),
15 Leaves hotel (CHECKS OUT) for European’s (Czechs) speech (= homophone indicator) (‘Czechs’ = ‘CHECKS’) on holiday (OUT) (CHECKS OUT),
16 A small sweet (PASTILLE) herb (dill) starting to go off (d) (‘dill’ without ‘d’ = -ILL-) in spread (paste = PAST-E) (PASTILLE),
17 Dealer (MERCHANT) produces small car (Mercedes = Merc) reduced (c) (‘merc’ without ‘c’ = MER-) for church (-CH-) worker (-ANT) (MERCHANT),
19 Small homely type (BEDSIT) of piece (bit = B-IT) about journalists (Eds = -EDS-) (BEDSIT),
20 Overthrow (TOPPLE) Crown (TOP-) with half the people (half the ‘people’ = -PLE) (TOPPLE),
23 State (= homophone indicator) faces up to (‘meets’ = ‘MEATS’) a number of cuts (MEATS).