Across:
1 Pleasantly rural (SYLVAN) school (S-) pupil (learner = -L-) consumed by dark blue (navy) boomerang (= reverse indicator) (navy = -Y-VAN) (SYLVAN),
4 No malice (no malice) working (= anagram indicator) (no malice = COAL MINE) in the pit (COAL MINE),
9 Frenchmen (Messieurs = -MM-) in business attire (SU-IT) (SUMMIT) for meeting (SUMMIT).
10 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),
12 Run a piece (run a piece) rewritten (= anagram indicator) (run a piece = EPICUREAN) by bon vivant (EPICUREAN),
13 Article (an = A-N) about fiction (-LIE-) (ALIEN) produced by foreigner (ALIEN),
14 Opens 4 across (opens ‘coal mine’ = COAL) with shopkeeper (MERCHANT) (COAL MERCHANT) – the one whose old energy business may be slack in part? (COAL MERCHANT),
18 At the start of 14 across (at the start of ‘coal merchant’ = COAL) abandons ship perhaps (SCUTTLES) (COAL SCUTTLES) with containers of fuel (COAL SCUTTLES),
21 Irish love (Irish ‘love’ = ‘grá’ = GRA-) going all around ship (going all around ‘ShiP’ = SP) (GRASP) to take it all in (GRASP),
22 Driver’s (drivers = GOLF CLUBS) car (GOLF) association (CLUB-) and school (-S) (GOLF CLUBS),
24 Most wanting (NEEDIEST) engineer (-E-) to stop working (-DIE-) in tree house (nest = NE-ST) (NEEDIEST),
25 Like the bear (URSINE) in Russia (RU), raised (= reverse indicator) (RU = UR-) on the borders of Sierra Leone (on the borders of ‘SIerra leoNE’ = -SINE) (URSINE),
26 Palestinians release plan (‘Palestinians’ release ‘plan’ = estiians = SANITISE) to debug The Authority, for instance (SANITISE),
27 To poke fun (tease = TEA SE-) at Turtle primarily (‘Turtle’ primarily = -T) (TEA SET) is so essential for the Mad Hatters! (TEA SET for the Mad Hatters Tea Party).
Down:
1 In retrospect (= reverse indicator), English (-E) school (-S-) in Crete (Crete = -E-TERC-) is openly subsidised (openly 'Subsidised' = S-) (SESTERCE) with old money (SESTERCE),
2 Turns up (= reverse indicator) Indian dress (sari = -IRAS) for prime minister (PM = -MP-) going to the foreign (‘the’ foreign/Spanish = el = LE-) (LEMPIRAS) capital in South America (capital/money in Honduras = LEMPIRAS),
3 Delta (-D-) that is (i.e. = -IE-) in Australia (AU = A-U) (ADIEU) and so long (so long/goodbye = ADIEU),
5 Ignored sitting on the naughty step perhaps (OUT IN THE COLD) exterior to igloo (OUT IN THE COLD),
6 Large (L-) furniture maker (-IKE A-) turning over (= reverse indicator) thousands of pounds (tons = -S NOT) (LIKE AS NOT)? Probably! (LIKE AS NOT),
7 Rhetorician loses heart (‘Rhetorician’ loses ‘heart’ = roicin = IRONIC) for what’s sarcastic (IRONIC),
8 Having one’s fill (EATING) of the chair (seating) losing the head (s) (‘seating’ without ‘s’ = EATING),
11 Most (w) new (new) (‘new’ without ‘w’ = NE-) clinical workers specialising on disorders in the waterworks (urinary system) (-UROLOGISTS) (NEUROLOGISTS) presumably studying why some have butterflies (nerves)? (NEUROLOGISTS),
15 The word coming out of Jersey (MOO- from Jersey cow) that it’s time (-T) for Spike (POINT) (MOOT POINT) is open to debate (MOOT POINT),
16 Words of congratulation (PLAUDITS) in post (P-) for adults (adults) running (= anagram indicator) (adults = -LAUD-TS) around island (-I-) (PLAUDITS),
17 One (I-) taken in by easiest (easiest) trick (= anagram indicator) (easiest = AS I SEE -T) (AS I SEE IT), in my opinion (AS I SEE IT),
19 Gladstone dismissed old (‘Gladstone’ dismissed ‘old’ = gastne = AGENTS) spies (AGENTS),
20 Dan leaves in advance (‘Dan’ leaves ‘in advance’ = ivance = CAVE IN) of sudden collapse (CAVE IN),
23 Worry (CAR-E) about sign (Victory sign = -V- sign) (CARVE) for a job in the butchers (CARVE).