Across:
1 Not the first (‘Not’ the first = n) to leave Italian port (Naples) (‘Naples’ without ‘n’ = -APLES) after (= position indicator) Mass (M-) (MAPLES) for members of The Acer family (MAPLES are members of the acer family of trees),
4 Seized (SNATCHED) revolutionary (= reverse indicator) colourful type (tan = -NAT-) caught (-C-) in outbuilding (shed = S-HED) (SNATCHED),
9 Accommodates The Grateful Dead perhaps (GRAVE-) with leader leaving (leader ‘Leaving’ = -L) (GRAVEL) the rock group? (GRAVEL),
10 Makes every effort (ATTEMPTS) to get top scorer (top ‘Scorer’ = -S) first (= position indicator) try (ATTEMPT-) (ATTEMPTS),
12 A trifle (TIPSY CAKE) of the rewards for waiting (rewards for waiting in a restaurant = TIPS-) for Yes (-Y) cover (CAKE) (TIPSY CAKE),
13 Song and dance (SCENE) witnessed at the gate (SCENE in The Gate Theatre),
14 Bar (LINE) for coaches (MANAGERS) (LINE MANAGERS) and those running the teams (LINE MANAGERS),
18 Man of Letters (SCHOLAR-) on Titanic (-SHIP-) from the start singlehandedly (from the start ‘Singlehandedly’ = -S-) (SCHOLARSHIPS) paid for opportunities for further learning (SCHOLARSHIPS),
21 Glamorous to release logs (‘glamorous’ to release ‘logs’ = amrou = AMOUR) of what’s essential to nice romance (what’s essential to Nice/France ‘romance’ = ‘love’ = AMOUR),
22 The director’s handling of the play (TREATMENT) performed in The Theatre (TREATMENT in hospital theatre),
24 Whip (= anagram indicator) out hanky (out hanky = THANK YOU) in response to what you might be given (THANK YOU),
25 Oral history (lore = LO-RE) about Light (ultraviolet / UV light = -UV-) (LOUVRE) Museum (LOUVRE),
26 Trick (CON-) on Greek island (-CRETE) (CONCRETE) for securing property (CONCRETE),
27 Made up (UNTRUE) with a foreign (‘a’ French = UN-) tenor (-T-) on French street (French ‘street’ = -RUE) (UNTRUE).
Down:
1 With great force (MIGHTILY), it’s ferocious (MIGHT-Y) around one (-I-) port (left = -L-) (MIGHTILY),
2 Work by The Abbey (PLAY-) writers (-PENS) (PLAYPENS) amusingly corralling childish types (PLAYPENS),
3 Hostile force (ENEMY) in unstable (= anagram indicator) Yemen (Yemen = ENEMY),
5 Idiot (nutter = NUT-TER) accommodating Frenchman (-M-), for example (e.g. = -EG-), finds love in Ireland (Irish ‘love’ = ‘grá’ = -GRA-) (NUTMEG GRATER) with the type that roughly adds a bit of spice? (NUTMEG GRATER),
6 Cherishes (TREASURES) rare set (rare set) built (= anagram indicator) (rare set = TREA-RES) around unfinished (b) replacement (‘sub’ without ‘b’ = -SU-) (TREASURES),
7 Dream (HOP-E-) about Progressive (-P-) Democrat (-D) (HOPPED) bounced along (HOPPED),
8 Is (-IS-) he (-HE-) arrested by policeman (Detective Sergeant = DS = D-S) (DISHES) seen coming out of the kitchen? (DISHES),
11 Writing (from the 3Rs = R-) ain’t (-AIN- T-) about Bob (bob/curtsy = -BOW) on short (e) course (‘route’ without ‘e’ = -ROUT) (RAINBOW TROUT) with colourful swimmer (RAINBOW TROUT),
15 Creative type (maker = M-AKER) outside ruck (ruck) all over the place (= anagram indicator) (ruck = -UCKR-) (MUCKRAKER) spraying dirt on others? (MUCKRAKER),
16 Behave yourself (GIVE OVER) with generous soul (giver = GIVE -R) around lover at the centre (‘lOVEr’ at the centre = OVE-) (GIVE OVER),
17 Problematic (= anagram indicator) satire (satire = AS IT -RE) about you and I (WE-) (AS IT WERE), in a manner of speaking (AS IT WERE),
19 Scheme (TACTIC) set up (= reverse indicator) for most of those (y) in Los Angeles (city) (‘city’ without ‘y’ = cit = -TIC), California (CA = -AC-) starting therapy (starting ‘Therapy’ = T-) (TACTIC),
20 Party (DO-) worker (man = -MA-N) crossing one (-I-) (DOMAIN) estate (DOMAIN),
23 Those initially (‘Those’ initially = T-) heard coming from the car (-HORN) (THORN) protecting Rose (THORN).