Across:
1 Don’t leave unescorted (‘dont’ leave ‘unescorted’ = uescre = SECURE) from stable (SECURE),
4 Urge pool (urge pool) model (= anagram indicator) (urge pool = PROLOGUE) to put this in a book at the start (PROLOGUE),
9 A subtle difference (NUANCE) to Mother Superior (nun = NU-N-) protecting a (-A-) church (Church of England = -CE) (NUANCE),
10 Issues (EDITIONS) upsetting (= reversal indicator) leaders of North of Ireland tourism (leaders of ‘North Of Ireland Tourism’ = noit = -TION-) divides Ulster, for one (Ulster = side = EDI -S) (EDITIONS),
12 Horseplay perhaps (POLO) after (= position indicator) drink (WATER) (WATER POLO) leads to game of pool? (WATER POLO),
13 A sad feeling (downer) coming off the summit (d) (‘downer’ without ‘d’ = OWNER) with title-holder (OWNER),
14 Those calming things down (PEACEKEEPERS) for 23 down (‘peace’ = PEACE-) and 20 down (‘keeper’ = -KEEPER-) at school (-S) (PEACEKEEPERS),
18 Pouring oil on troubled waters (PEACEKEEPING) is the job of 14 across (it’s the job of ‘peacekeepers’ to do the PEACEKEEPING),
21 Closing the surveillance (closing the ‘surveillancE’ = E-) outlet (-VENT) (EVENT) is a news item (EVENT),
22 A real open (a real open) question (= anagram indicator) (a real open = AEROPLANE) for highflyer (AEROPLANE),
24 The old man (pops = POP -S) grasps air (SONG-) (POP SONGS) in record numbers (POP SONGS),
25 Saucy type (TAHINI) upsets (= reversal indicator) bowler (hat = TAH-) in (-IN-) taking second wicket (second ‘wIcket’ = -I) (TAHINI),
26 Inadequate (DERISORY) single (-I-) in New (= anagram indicator) Order’s (orders = DER-SOR-) last anthology (last ‘anthologY’ = -Y) (DERISORY),
27 State (= homophone indicator) spy (‘see’ = ‘sea’ = SEA) found in Mayo (EGG found in mayonnaise) (SEA EGG) with urchin (SEA EGG).
Down:
1 Searched for bugs (swept = S-WEPT) welcoming a foreign (a = -UN-) head of spies (head of ‘Spies’ = -S) (SUN-SWEPT) enjoying a hot spot (SUN-SWEPT),
2 Applause (CLAP-) for trick (-TRAP) (CLAPTRAP) is absurd (CLAPTRAP),
3 Necromancer dismisses con men (‘necromancer’ dismisses ‘con men’ = racer = RACER) with runner at the track (RACER),
5 Very popular (RED-HOT) game (POKER-) at school (-S) (RED-HOT POKERS) and in the garden (RED-HOT POKERS),
6 Release without any charge (LET GO FREE) in blocks (Lego = LE- GO) around square? (-T square) That’s priceless! (FREE) (LET GO FREE),
7 Reacts to bad jokes (GROANS) by playing (= anagram indicator) organs (organs = GROANS),
8 Guard (guard/take care = ENSURE), engineer (E-), nurse (nurse = -NSURE), butcher? (= anagram indicator) (ENSURE),
11 Individual (-ONE-) accommodated in my (M-Y) conversion (CHANGE-) is finding resistance (-R) (MONEY CHANGER) from one of those exchanging notes for the coppers (MONEY CHANGER),
15 Criminal (= anagram indicator) to silence (to silence = ELECTIONS) those deciding who’ll get into office (ELECTIONS),
16 Settle (FINALISE) one (-I-) consumed by big exams (finals = FINAL-S-) with note (music note -E) (FINALISE),
17 In accord (AGREEING) with reggae in (reggae in) medley (= anagram indicator) (reggae in = AGREEING,
19 On the phone (KEYPAD) to sweetheart (‘sweet’ heart = -E-) in Kerry (K-Y-) with writer’s block? (-PAD) (KEYPAD),
20 Look (peek = KEEP-) up (= reversal indicator) English (-E-) Royal (-R) (KEEPER) in The Guardian (the guardian = the KEEPER),
23 Sample (sample/section = piece) on the radio (= homophone indicator) (‘piece’ = ‘PEACE’) of Law and Order (PEACE).