Across:
8 Pledge (COMMIT) to most of those opening 15 across (most of those opening ‘commits to memory’ = COMMIT),
9 It’s about (RE-) 21 down (‘heated’ = -HEATED) (REHEATED) put back on the range (REHEATED),
10 Some populist unfortunate (some ‘populiST UNfortunate’ = STUN) from Knock (knock/hit = STUN),
11 Take a hike locally (GET KNOTTED), for example (e.g.), around (= reverse indicator) (e.g. = GE-) square (-T square) and get twisted (KNOTTED) (GET KNOTTED),
12 Head of thyme (head of ‘Thyme’ = -T-) found among fresh (= anagram indicator) herbs (herbs = BER-HS) (BERTHS) in beds (BERTHS),
14 Shakes (DISTURBS) Diana’s (Di’s = DIS-) very dry champagne (brut) served up (= reverse indicator) (brut = -TURB-) (DISTURBS) with starter of scallops (starter of ‘Scallops’ = -S) (DISTURBS),
15 Keeps in mind (COMMITS TO MEMORY) more than one of those in 7 down, in a similar sense (more than one ‘remember’ = remembers, in a similar sense = COMMITS TO MEMORY)
18 Crucial (KEY) sources of Vitamin C (LIMES) (KEY LIMES) in fruit (KEY LIMES),
20 Spud (MURPHY) serves up (= reverse indicator) drink (rum = MUR-) with a measure of acidity (-PH-) for Fourth of July (fourth of ‘julY’ = -Y) (MURPHY),
22 Project leader (PRIME MOVER) gets to brief (to brief = PRIME) Man-with-a-Van (MOVER) (PRIME MOVER),
24 Set off Maltese (‘set’ off ‘Maltese’ = male = MEAL) or Indian? (Indian MEAL),
25 Box (SPAR-) with crackers (= anagram indicator), brie (brie = -E RIB) (SPARE RIB) – was that pulled out for the First Lady? (SPARE RIB),
26 English (E-) beers (ales = -A-LES), including golf (-G-) (EAGLES), welcomed by the swingers, of course (EAGLES welcomed by the golfers).
Down:
1 Foams out of false bottom (‘foams’ out of ‘false bottom’ = lebtto = BOTTLE) in plastic container (BOTTLE),
2 Writing on the wall (OMEN) up (= reverse indicator) in Stone Mountain (in ‘stoNE MOuntain’ = nemo = OMEN),
3 Out of order (= anagram indicator) gates (gates = STAGE) ready to go (SET) (STAGE SET) from play area (STAGE SET),
4 Turns up (= reverse indicator), it’s wrong according to the judge (tort = TROT) to run (TROT),
5 Church (CH-) workers (-ANTS) (CHANTS) produced music for The Abbey (CHANTS),
6 Salt (salt) solution (= anagram indicator) (salt = LAST) and aggregate (SUM-) from French sea (French ‘sea’= -MER) (LAST SUMMER) produced back in July, a year ago (LAST SUMMER),
7 Bear in mind (REMEMBER) one of the team (-MEMBER) is after (= position indicator) one of those flipping (= reversal indicator) Wards (one ward = er = RE-) (REMEMBER),
13 Training hub for The Boys in Blue (TEMPLEMORE Garda Training College) and for those worshipping their idols (TEMPLE-), to a greater extent (-MORE) (TEMPLEMORE),
14 Part of Somerset advertisement (part of ‘somerSET ADvertisement = setad) for return (= reverse indicator) (setad = DATES) of fruit (DATES),
16 It’s beyond mature (OVERRIPE) for Romeo (lover) to remove intro (l) (‘lover’ without ‘l’ = OVER-) on last letters (-RIP-) to sweetheart (‘swEet’ heart = -E) (OVERRIPE),
17 You’re unlikely to forget those (MEMORIES) in short (m) viral phenomenon (‘meme’ without ‘e’ = MEM-) with British politicians (Tories) losing the head (t) (‘Tories’ without ‘t’ = -ORIES) (MEMORIES),
19 Computer storage (MEMORY) at the end of 15 across (at the end of ‘commits to memory’ = MEMORY),
21 Harvey left heavyhearted (‘harvey’ left ‘heavyhearted’ = heated = HEATED) and inflamed with passion (HEATED),
23 Unadvisable to get rid of landaus (‘unadvisable’ to get rid of ‘landaus’ = vibe = VIVE) or that’s the vague feeling (VIBE),
24 Sleight of hand (MAGIC) from wise guys (MAGI-) and leading criminal (leading ‘Criminal’ = -C) (MAGIC).