Across:
8 Miss Piggy (sow = SO W-) takes pork pie (pork pie = -HAT) (SO WHAT) – and why would that be significant? (SO WHAT?),
9 Casual (CARELESS) mini (CAR-) with elegant top (‘Elegant’ top = -E-) is reduced (-LESS) (CARELESS),
10 It’s more than cool (COLD) from poster child dismissing hipster (‘poster child’ dismissing ‘hipster’ = ocld = COLD),
11 Brought to mind (REMEMBERED) by one of those from the club (-MEMBER-) in Rush (rush = RE-ED) (REMEMBERED),
12 College (C-) in Haiti (Haiti) destabilised (= anagram indicator) (Haiti = TAI -HI) (TAI CHI) by exercise that encourages arty type of go-slow? (TAI CHI martial art),
14 Doctor (doctor/tamper = MESS WITH) produces dog’s dinner (MESS) to humour (WIT-) handler initially (‘Handler’ initially = -H) (MESS WITH),
15 It’s similar to 8 across (similar to ‘so what’ = COULDN’T CARE LESS) and 10 across (‘cold’ = CO-LD-) consuming, you (you) hear (= homophone indicator) (‘you’ = ‘u’ = -U-), good books (New Testament = NT) by 9 across (‘careless’ = CARE LESS) (COULDN’T CARE LESS),
18 Design (= anagram indicator) sofa (sofa = AS OF) on blackboard (slate) missing school (s) (‘slate’ without ‘s’ = LATE) (AS OF LATE) recently (AS OF LATE),
20 Check out (ASSESS) ship (-SS-) in wild (= anagram indicator) seas (seas = A-ESS) (ASSESS),
22 Panettone (BREAD) and butterscotch (butterscotch = SAUCE) (BREAD SAUCE) served at Christmas, for example (BREAD SAUCE),
24 The limits to well-educated (the limits to ‘WEll-educatED’ = WEED) Scarlet Pimpernel (‘scarlet pimpernel” is a weed = WEED),
25 Timetable (SCHEDULE) for revolutionary (-CHE-) head of department (head of ‘Department’ = -D-) at university (University of Limerick = -UL-) in south-east (SE = S-E) (SCHEDULE),
26 A fair amount of (half) turf (turf) (half of ‘TUrf’ = TU-) and borders (-LIPS) (TULIPS) for flowers (TULIPS).
Down:
1 Characters in Jumbo got away (characters in 'jumBO GOT Away' = BOGOTA) from country's capital (BOGOTA is capital of Colombia)
2 Charting hills and defined summits (‘Charting Hills And Defined’ summits = CHAD) in Africa (CHAD),
3 Is Grant’s (is Grant’s) doctor (= anagram indicator) (is Grant’s = STAR SIGN) Leo? (STAR SIGN)
4 Rip-off (SCAM) covered up by Thomas Campion (covered up by ‘thomaS CAMpion’ = SCAM),
5 Fits up local (‘fits up’ local = FRAMES) possibly sold out by the opticians (FRAMES),
6 Travels at night (SLEEPWALKS) with 21 down (‘sleeps’ = SLEEP-S) around 24 down (‘walk’= -WALK-) (SLEEPWALKS),
7 Harmful material (ASBESTOS) in Belgium (-BE-) found on street (-ST-) starting off (starting ‘Off’ = -O-) and pocketed by fool (ass = AS-S) (ASBESTOS),
13 10 across (‘cold’ = COLD) and 5 down (‘frames’ = FRAMES) (COLD FRAMES) presumably keeping those in the nursery warm (COLD FRAMES),
14 Chiastolite (MACLE) concealed by Annabel Campbell taking a turn (= reverse indicator) (concealed by ‘annabEL CAMpbell’ = elcam = MACLE),
16 Caught (-C-) among wild (= anagram indicator) turbots (turbots = OBSTRU-T) (OBSTRUCT) found in hamper (hamper/impede = OBSTRUCT)
17 Some American university governors (regents = RE-GENTS) welcoming head of academy (head of ‘Academy’ = -A-) (REAGENTS) employed in the science laboratory (REAGENTS),
19 In that studio (in ‘thAT STUDio’ = AT STUD) prepared to be a father to the Arabians (horses) (AT STUD),
21 Strips (peels), retiring (= reversal indicator) (peels = SLEEP-) and closing eyes (closing/last letter ‘eyeS’ = -S) (SLEEPS), drops off (SLEEPS),
23 Fraudster’s release? Drafts (‘fraudsters’ release ‘drafts’ = uers = USER) for consumer! (USER),
24 Tramp (WALK) knocks back (= reverse indicator) the bar (law = WAL-) cocktail (‘cocK’ tail = -K) (WALK).