Across
8 Pavarotti (tenor = T) had mashed (= anagram indicator) tuber (TUBER) with this (=BUTTER)!
9 At last Greta will be alone (ref. Garbo, at last = ARBO) in this truly (= REAL) sylvan place (= ARBOREAL).
10 A twist (= EDDY) revealed by (indicates hidden word) 18 across. (= tEDDY boy).
11 A cheesy snack (= QUESADILLA) to have with ale in squalid surroundings (= anagram of ALE and SQUALID).
12 Eel squirms in bag (‘squirms’ = anagram indicator, EEL and BAG) on naturalist’s (i.e. Charles Darwin) vessel (= BEAGLE).
14 Spanish gent (= DON) turns pale (‘turns’ indicates anagram, ‘pale’ = WHITE) - he’s finished (=DONE WITH).
15 Experiment at home, perhaps? (‘home’ = DOMESTIC, ‘experiment’ = science).
18 Rebellious (= anagram indicator and ref. ‘Teddy Boy’) Toby dyed (= TOBY DYED) his long jacket (ref. Teddy Boy style)!
20 Mark (= INK) the colour of the blocks here (= JET) for printing (=INK-JET).
22 In small bites (= FINGER FOOD), 15 across (= ‘domestic science’) might teach you to wash hands before you do this (i.e. ‘finger food’).
24 Shortening (= SUET) Tuesday (‘shortening Tuesday’ = TUES)!
25 It will happen once (= INSTANCE) this month (= INST), by chance not in church (remove ‘CH’ (i.e. ‘church’) from CHANCE = ANCE).
26 A 4 down (‘cake’) to use to clean 23 down (‘oven’), perhaps? (= SPONGE)
Down
1 It’s horrific to do this to the blood (= CURDLE) but around (= anagram indicator) fifty (=L) will be healed (= CURED).
2 Remain (= STAY) on the road (ref. ‘street’ = ST) on the outskirts of Albany (‘outskirts of Albany’ = AY).
3 I hear (homophone indicator) the weirdo (= ‘freak’) used go (= ‘went’) there pretty often (= FREQUENT).
4 Sweet enough (= CAKE) to relieve (‘relieve’ = remove letters) Bach of his backache (remove ‘Bach’ from ‘Backache’ = KACE).
5 Get (= OBTAIN) the boat in awkwardly (‘boat in awkwardly’ = anagram of BOAT IN).
6 Know that, without his ring (= KNOW without O = KNW), the diamond-seller (CARTIER) ran amok (= anagram indicator) and went completely off the rails (= TRAIN WRECK).
7 Moves well (= BALLETIC), has balance to start with (BAL-) and is at least somewhat athletic (-LETIC).
13 Mix (= anagram indicator) 24 across (=SUET) with dry egg (DRY EGG) for one who’ll eat anything and everything (= GREEDYGUTS).
14 Biodegrade cayenne pepper inside (‘inside’ indicates hidden word, ‘biodegraDE CAYenne pepper’) this (= DECAY)!
16 Scatter (= anagram indicator) ten (TEN) around (O) one (I) bed (BED), in observance of the rules (= OBEDIENT).
17 Youthful (= CHILDISH) Achilles had no seal (‘Achilles’ without ‘seal’ = CHIL) on his plate (=DISH).
19 It’s tiresome (= BORING) for a bee (=B) to go round (= O) and round (= RING).
21 The gamekeepers won’t speak (remove ‘speak’ from ‘gamekeepers’ = GMEEER) about it (= anagram indicator) but it will come out (= EMERGE).
23 Never start up (= VEN) with a ring (= O) of fire here, perhaps (= OVEN).
24 Plummy (= SLOE), and said to be (= homophone indicator) a bit sluggish (‘slow’, i.e. SLOE).