Solution to 21A has been deliberately left unsolved and is to be answered by a non-regular/novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Began to sing in lorry, enthralled by drink (6,2) STRUCK UP {S{TRUCK}UP}
5 Prepare for major housing amendment (6) REFORM [T]
10 Guide in rough terrain (7) TRAINER*
11 Pep up in final, making a comeback after catch (7) HEARTEN {NET<=}<=>{HEAR}
12 A little lamp in back row's illuminated again (5) RELIT {Lamp} in {TIER<=}
13 Racing with caution on essentially dangerous circuit (9) CAREERING {CARE}{d..gEr..s}{RING}
14 What bats will do to remain united (4,8) HANG TOGETHER [C&DD]
18 Emphatic about criminal being unstable (12) INCONSISTENT {IN{CON}SISTENT}
21 Disastrous match! Coming to grips with primarily shocking collapse (9) D?S?E?A?R ()
23 Rude lady breaks heart, goes round with model (5) BLUNT {NU{L}B<=}{T}
25 Small amount of dope after a little meth in endless party (7) SMIDGEN {GEN}<=>{S{Meth}IDe}
27 Despicable boy advanced to ensnare good girl (8) BEGGARLY {B}{E{G}{G}ARLY}
DOWN
1 Lampoon American involved in agitation on base (6) SATIRE {S{A}TIR}{E}
2 Truly perk up after consuming ecstasy (6) REALLY {R{E}ALLY}
3 Pressurise with rooks on column (9) CONSTRAIN {CONS}{TRAIN}
4 Unidentifiable bag in enclosure, suspicious (14) UNRECOGNISABLE*
6 Remove saree naughtily (5) ERASE*
7 Completely unacceptable massage by houseguest on vacation (8) OUTRIGHT {OUT}{RIG}{Ho...sT}
8 Cribs about primarily arrogant administrators (8) MANAGERS {MAN{Ar...t}GERS}
9 Secures fantastic sandwiches, tart and bit of tiramisu for dessert (9,5) CHARLOTTE RUSSE {SECURES}* over {HARLOT}{Ti...u}
17 Blizzard in sector, I'm worried (3,5) ICE STORM*
19 Need enormous runs to win final of competition (6) HUNGER {HUGE}{R} over {c...oN}
20 Error to retain tenor, extremely grumpy and mean (6) STINGY {S{T}IN}{GrumpY}
22 Contestant wanting special watch (5) ENTRY sENTRY
Reference List
Drink = SUP, Final = NET, Criminal = CON, Lady = L, Model = T, Dope = GEN, Boy = B, Good = G, Girl = G, American = A, Base = E, Ecstasy = E, Rooks = CONS, Runs = R, Tenor = T, Special = S
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