Across:
1 So long (so long! = BYE-BYE) as you hear this (= homophone indicator) repeatedly when the price is right (‘buy-buy’ = ‘BYE-BYE’),
4 Exceptional (SPECIAL-) salad starter (‘Salad’ starter = -S) (SPECIALS) in the restaurant can be different every day (SPECIALS),
9 Maintains a high level (TOPS UP) opening pump (opening ‘Pump’ = -P) behind (= position indicator) new (= anagram indicator) spout (spout = TOPS U-) (TOPS UP),
10 Goes up (= reversal indicator) road (MI = IM-) to fully developed (-MATURE) (IMMATURE) green (IMMATURE),
12 Playing (= anagram indicator) Garth Peel (Garth Peel = TELEGRAPH) in The Wire (TELEGRAPH),
13 Crew from Greenwich (‘crew’ from ‘Greenwich’ = genih = HINGE) on the door (HINGE),
14 Can’t keep mouth shut (BREAK SILENCE) to most of those in 25 across (most of ‘breaks’ = BREAK) on the radio? (= homophone indicator) Quite! (‘quite’ = ‘quiet’ = SILENCE) (BREAK SILENCE),
18 They rank the performances (LEAGUE TABLES) of association (LEAGUE) used by mathematicians (TABLES) (LEAGUE TABLES),
21 Rake in from painkillers (‘rake in’ from ‘painkillers’ = pills = PILLS) sold by the chemist (PILLS),
22 Flexible (VERSATILE) relatives (relatives) all over the place (= anagram indicator) (relatives = VERSATILE),
24 Scurried (ran) up (= reverse indicator) (ran = NAR-) to bed (bed = -COT-) with current (-I-) cold (-C) (NARCOTIC) with the equivalent of one of those in 21 across (equivalent of one of those ‘pills’ = NARCOTIC),
25 Snaps (BREAKS) baker’s (bakers) crackers (= anagram indicator) (bakers = BREAKS),
26 Gags (SILENCES) produced by those at the end of 14 across (at the end of ‘break silence’ = SILENCE-) on bender (-S bend) (SILENCES),
27 Lists of figures (TABLES) closing 18 across (closing ‘league tables’ = TABLES).
Down:
1 Places to get one’s ducks in a row (BATHTUBS) where everyone can come clean (BATHTUBS),
2 Asked by mistress to leave (EXPELLED) as it turned out? (EXPELLED)
3 The issue (YOUNG) in the early years (YOUNG),
5 Suspect (= anagram indicator) Ethel (Ethel = -HLETE-) is surrounded by babies (babies = pampers = PAMP-ERS) (PAMPHLETEERS) of political writers (PAMPHLETEERS),
6 Clips (clips/cuts ‘t’) of three conversations (chat x 3) (‘chat + chat + chat without ‘t’ = CHA-CHA-CHA) in the dance hall (CHA-CHA-CHA),
7 Malthusian dismisses this (‘Malthusian’ dismisses ‘this’/ without ‘this’ = maluan = ALUMNA) by female graduate (ALUMNA)
8 Dramatic divisions (SCENES) leads to public spats (SCENES),
11 Just saying (HAVING A VOICE) this is speaking up for oneself (HAVING A VOICE),
15 Doesn’t raise head above parapet (KEEPS DOWN) in castles (KEEPS) in Ulster (DOWN) (KEEPS DOWN),
16 ALALLL? (‘all’ is in ‘all’ = ‘alALLl’ = ALL IN ALL) That’s considering everything! (ALL IN ALL),
17 Slanders, in a literary sense (ASPERSES), local fools (asses = AS-SES) holding a gent (‘a gent’ = ‘agent’ = rep) up (= reverse indicator) (rep = -PER-) (ASPERSES),
19 Uses up the budget (SPENDS) for writer (-PEN-) and director (-D-) in Titanic, for example (RMS Titanic steamship = S-S) (SPENDS),
20 More than one (PLURAL) place (PL-) with Russian flower (flower/river Ural = -URAL) (PLURAL),
23 Loses the head (p) on drink (‘port’ without ‘p’ = -ORT-) and goes into Alcoholics Anonymous (A-A) (AORTA) – will the cardiologist be able to handle it? (AORTA).