Across:
1 Daphne and others (NYMPHS) in American city (New York = NY-) with leader of The House (lead of the 'House' = -H-) welcomed by foreign politicians (MPs = -MP-S) (NYMPHS),
4 It’s a revelation (EPIPHANY) to one with great expectations (-PIP- is the one with great expectations in Charles Dickens’ Great Expectations) trapped by wild (= anagram indicator) hyena (hyena = E-HANY) (EPIPHANY),
9 Mark (M-) and Noel’s (noels = -ELONS) salad (= anagram indicator) (MELONS) with large fruits (MELONS),
10 Unable to open up (AIRTIGHT) to couple (pair) losing the head (p) (‘pair’ without ‘p’ = AIR-) with local drunk (‘drunk’ = -TIGHT) (AIRTIGHT),
12 Revolutionary (RED) haircut (MULLET) (RED MULLET) for swimmer (RED MULLET),
13 Clip (n) of Nirvana (‘heaveN’ without ‘n’ = HEAVE) produced by local Yank? (locally ‘yank’ = HEAVE),
14 It’s for airing (CLOTHES HORSE) jumpers (CLOTHES) found in the stable (HORSE) (CLOTHES HORSE),
18 Set-up for anyone getting into drugs trade (drugs trade = JOINT VENTURE) business incorporating the Camorra and ’Ndrangheta, for example (JOINT VENTURE),
21 A (A-) revenue (Revenue Online Service = -ROS-) from energy (-E) (AROSE) came to light (AROSE),
22 Cherishes (TREASURES) rare set (rare set) built (= anagram indicator) (rare set = TREA-RES) around unfinished (b) replacement (‘suB’ without ‘b’ = -SU-) (TREASURES),
24 Drawing power (CHARISMA) from amulet (CHAR-M) around one (-I-) from South (-S-) Africa? (-A) (CHARISMA),
25 Opens diner, eatery (opens ‘Diner Eatery’ = DE-), outlet (-DUCT) (DEDUCT) and takeaway (take away = DEDUCT),
26 Final (LAST), final warning (final ‘warninG’ = G-) for snaky type (-ASP) (LAST GASP) sounds like a touch of desperation (LAST GASP),
27 Learner (-L-) in right (S-IDE- = position to right/left) school (-S) (SLIDES) found in the playground (SLIDES).
Down:
1 Salad (= anagram indicator) menu (menu = NUME-) that's spicy (-RACY) (NUMERACY) is essential for those working on their figures (NUMERACY)
2 Played by a folk musician (MELODEON) in short (t) run (melt) (‘melt’ without ‘t’ = MEL-) on old Greek stage (-ODEON) (MELODEON),
3 A religious type (HINDU) of independent (-IND-) behind hospital (H-) and university (-U) (HINDU),
5 Convictions (POINTS OF VIEW) handed out for breaking speed limit (POINTS) going around (O-) French (-F) landscape (VIEW) (POINTS OF VIEW),
6 Does one’s bit (PITCHES IN) for detective (PI-) initially captured (initially ‘Captured’ = -C-) in article (the = -T-HE-) associated with wrongdoing (sin = -S IN) (PITCHES IN),
7 Iranian neighbour (AFGHAN) provides a (A-) party (Fine Gael = -FG-) for Chinese ethnic group (-HAN) (AFGHAN),
8 Local incessant talk (YATTER) relates to Treaty (treaty) negotiation (= anagram indicator) (treaty = YATTER),
11 Type of chateau (CLOS-) in (-IN-) German capital (‘German’ capital = -G) with bats (= anagram indicator), mites (mites = TIMES) (CLOSING TIMES) and types that end trading in 20 down, for instance (CLOSING TIMES end trading in ‘locals’),
15 Sucking up on the dance floor perhaps (HOOVERING), all backing (= reverse all indicator) Mercury (Hg = H-G) touring Ulster (NI = -IN-) with Father (Rev = -VER-) Ring (-O-) leading the orchestra (leading the ‘Orchestra’ = -O-) (HOOVERING),
16 Endless (f) foam (‘surf’ without ‘f’ = SUR-) on pints of stout (-ROUND) (SURROUND) in Ring (SURROUND),
17 Network (WEB-) refers to (cites) reports (= homophone indicator) (‘cites’ = ‘sites’ = -SITES) (WEBSITES) of destinations for surfers (WEBSITES),
19 Friend (pal = PA-L) admits writing (one of the three Rs = -R-) to church (Church of England = -CE-) (PARCEL) – what would you expect from a Man of Letters? (PARCEL),
20 Bars (LOCALS) those who are familiar with the area (LOCALS),
23 First morsel (first ‘Morsel’ = -M-) consumed by hawk (sell = S-ELL) (SMELL), in a sense (SMELL).