Solution to 21A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
ACROSS
8 Antlered hybrid shrugs off insect — lady’s animal (4) DEER antlERED*
9 Omissions excepted, writes down exactly, like, what Ali Baba declared (4,6) OPEN SESAME {O{PENS}E}{SAME} 'Omissions excepted' OE, from E&OE? Containment indicator missing
10 Passes letter bills (6) ENACTS {EN}{ACTS}
11 Air-conditioned housing as unreservedly packed as a musical performance that goes swimmingly? (8) AQUACADE {A{QUA}C}ADE Anno for ADE not clear (Addendum - {A{QUA}C}{lADEn} - See comments)
12 Held firm, overturning award — that’s a little drastic (8) OCCUPIED {CO<=}{CUP}{IE}{Dr...c}
14 Head of department lops everyone’s heads after new HOD provides no kind of resolution (6) NOODLE {Of}{De...t}{Lops}{Ev...e}<=>{N}O Anno for O not clear (Addendum - {Of}{De...t}{Lops}{Ev...e}<=>{N}{hOd} - See comments)
16 Other wise elders organised no doctor (4) ELSE ELdErS*
17 Perhaps vegetable’s regurgitated before shedding its kernel (5) MAYBE {YAM<=}{BeforE}
18 Offer leeway (4) GIVE [DD]
19 Deviously entrap, make headway? (6) TREPAN*
21 Make use of return ticket to take model out, tour around Spain without any escort (8) Y?U?S?L? (Addendum - {Y{tOUR}{ES<=}LF<=} - See comments)
23 Cut joke after second cut (5,3) SPARE RIB {RIB}<=>{S}{PARE}
27 Best car hut invented since sliced bread? (10) BRUSCHETTA* Definition adequate?
DOWN
1 Common cure not available in any form — contained in deformed larva lacking blood type (10) VERNACULAR {CURE+NA}* in {LARVa}*
2 Break free suddenly, pretend you’re captured (8) FRACTURE {FR{ACT}{UR}Ee}
3 French goods with artificial intelligence? Japanese shoot! (6) BONSAI {BONS}{AI} Good is BON but Goods/Bons! Is Bonsai a shoot??
4 Second season will be in France (4) SERA {S}{ERA}
5 Coming out is nuanced — no doubts to begin with; left out in society (8) ISSUANCE {IS}{nUANCEd +S}*
7 In the middle of a bid, I’m asked to return cover (4) AMID {A}{I'M<=}D Anno for D pending (Addendum [T] - See comments)
13 Doctor Murthy finally ends drinking liqour, to start like a teetotaller (5) DRYLY {DR}{m...hY}{f...lY} over {Li...r}
15 Jackpot! Almost winning five elections — a third promise (4,6) LOVE LETTER {LO{V}{ELEctions}TTERy} Definition okay?
17 Excited man finds love, losing independence — rich year for his single-man regime? (8) MONARCHY {M{0}NA*}{RiCH}{Y}
18 Boy, girl running back-to-back laps regularly near sports department — in full throttle (3,5) GAS PEDAL {LAD}{G}<= over {lApS}{PE} What's the contaiment indicator? Definition vague?
20 Term right in the middle of term (6) PHRASE {PH{R}ASE}
22 Uncle almost had an hour out in the nude (6) UNCLAD {UNCLe}{hAD}
24 Rope around hole (4) PORE*
25 Snack chewed with drug (4) BITE {BIT}{E}
Reference List
Lady = L, Letter = EN, New = N, As = QUA, Firm = CO, Model = T, Second = S, Society = S, Independence = I, Year = Y, Girl = G, Sports department = PE, Right = R, Hour = H, Drug = E