Across:
8 Party (DO-) dress (mini) isn't finished (i) ('mini' without 'i' = -MIN-) to go with old (-O) (DOMINO) mask for masquerade (DOMINO mask),
9 Sounds like (= homophone indicator) Ryle (‘Ryle’ = ‘rile’ = OFFEND-) Ward (ward = -ER) (OFFENDER) is the culprit (OFFENDER),
10 Crime bosses (DONS) on the outskirts of Dorset Downs (on the outskirts of ‘DOrset dowNS’ = DONS),
11 It’s Apt (it’s apt = FITTING) in France where (French ‘where’ = OU-) contact at the centre (‘conTact’ at the centre = -T) (FITTING OUT) is supplying the necessary fixtures (FITTING OUT),
12 Upsets (= reversal indicator) bar (rod = DOR-) crowd (-SET) (DORSET) in England (DORSET),
14 What highly productive sketcher did (DREW LOTS) attracted a crowd (DREW LOTS),
15 Fellow (DON-) for time (T) finds most of those in 24 across (most of ‘kids’ = KID) going to Euros fly (Euros fly) direct? (= anagram indicator) (Euros fly = YOURSELF) (DON’T KID YOURSELF) Dream on! (DON’T KID YOURSELF),
18 Is less generous (STINGIER) to tigers in (tigers in) wild (= anagram indicator) (tigers in = STINGIER),
20 Record (Extended Play record = -EP-) by tenor (-T-) included in collection (set = S-ET) (SEPTET) – one of seven (SEPTET),
22 Employs men of letters (POST OFFICE) from 4 down (‘post’ = POST) and 5 down (‘office’ = OFFICE) (POST OFFICE),
24 Part of the fatuous diktat (part of the ‘fatuouS DIKtat’ = sdik) dragged up (= reverse indicator) (sdik = KIDS) by The Young Ones (KIDS),
25 Leaving (GOING-) one of those in Trinity (one in The Trinity: Father, Son and Holy Spirit = -S ON) (GOINGS-ON) synonymous with dodgy behaviour (GOINGS-ON),
26 Rush (dash = DA-SH) around at home (in = -NI-) serving up (= reverse indicator) (DANISH) something for breakfast (DANISH pastry).
Down:
1 Concerto Grosso lacks songster ('concerto grosso' lacks 'songster' = cocoro = ROCOCO) – the one that's theatrically ornate at the end of the Baroque movement (ROCOCO),
2 Fail to catch (MISS) what borders on midsummer madness (what borders on ‘MIdsummer madneSS’ = MISS),
3 The Paper covering the match (wedding CONFETTI) is thrown by those United supporters? (CONFETTI),
4 Opts (opts) out (= anagram indicator) (opts = POST) of appointment (POST),
5 Position (position/post = OFFICE) of rotten (OFF-) spire at the centre of (‘spIre’ at the centre of = -I-) church (-CE) (OFFICE),
6 Comes close to love (SNUGGLES UP) part of the bar (SNUG-) that’s great (-G-) for uplifting (= reverse indicator) function (use = -ES U) in backward (= reverse indicator) place (PL = -L- -P) (SNUGGLES UP),
7 Unholy (= anagram indicator) alert, but (alert but = REBUTTAL) that can’t be for the Confirmation! (not confirmation = denial / REBUTTAL),
13 Light metal (tin = -T IN) and sulphur (S-) found in deposit (set = SE- -T-) and bones at the centre (‘bONEs’ at the centre of = -ONE) (SET IN STONE) of Excalibur, presumably (SET IN STONE),
14 Describes wine (DRY- wine) that’s about (re = -ER) to be returned (= reverse indicator) (DRYER) produced by one that’s full of hot air (DRYER),
16 Conventional (ORTHODOX) alternative (OR-) to hot (hot), wild (= anagram indicator) (hot = -THO-) party (-DO-) for mystery man (-X) (ORTHODOX),
17 Rocky (UNSTEADY) is due a fall but not now (UNSTEADY),
19 Overwhelm (INFEST) with finest (finest) rum (= anagram indicator) (finest = INFEST),
21 Turns up (= reverse indicator), it’s time (-T) to take notice (see = -E-ES-) around Donegal (DL = -LD-) (ELDEST) of the first to arrive (ELDEST),
23 Banks increasing returns (banks ‘INcreasing returNS’ = INNS) for the locals (locals/bars = INNS),
24 Type of bed (KING bed) for some livestock in Germany (some ‘livestocK IN Germany’ = KING).