Solution to 3D 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.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Master dwelt on awful accident in nuclear reactor (8) MELTDOWN {M}{DWELT+ON}*
5 Extract from ripe cacao that's used to induce vomiting (6) IPECAC [T]
10 Mysterious moans in stately home (7) MANSION*
11 Wine merchant beginning to visit model in secret (7) VINTNER {Vi..t}{IN{T}NER}
12 Look at hunk, ultimately sexy (5) STUDY {STUD}{sexY}
13 Thievish con lures a drunk (9) LARCENOUS*
14 Cynthia baked crackers some time ago (4,2,3,3) BACK IN THE DAY*
18 Inadequate actions that pint drinkers avoid? (4,8) HALF MEASURES [C&DD]
21 Love to cuddle over essentially comfortable mat in front of fireplace (9) HEARTHRUG {HEART}{H{c...oRt...e}UG}
23 Ruffle Republican in charge (5) FRILL {F{R}ILL}
25 Invalidate union with sick, regressive fellow in New York (7) NULLIFY {N{U}{ILL<=}{F}Y}
26 Provides vehicles to tour Tenerife on vacation (6) CATERS {CA{Te...fE}RS}
27 Extremely wealthy youth at premiere consuming ecstasy in fact is naive (4-4) DEWY-EYED {We...hY}{Yo..h} over {E} in {DEED}
DOWN
1 Starts to malign inept member, very upset about bloomer (6) MIMOSA {Ma...n}{In..t}{Me...r}{SO<=}{A}
3 Raised millions to acquire secluded place producing milk (5,4) D?I?Y ?A?M ()
4 New tea maker with new lid worked very well (4,4,1,5) WENT LIKE A DREAM {TEA+MAKER+NEW+LID}*
6 British coppers in compound caught rogue finally (5) PENCE {PEN}{C}{r..uE}
7 Bond interrupts criminals making agreements (8) CONCORDS {CON{CORD}S}
8 Bows and arrows at first removed from terribly rusty case (8) CURTSEYS {RUSTY+CaSE}*
9 Go nude in revel, cavorting after consuming cocaine in excess (14) OVERINDULGENCE {GO+NUDE+IN+REVEL+C}*
15 Hydrogen and nitrogen extracted essentially in a careful manner (9) HEEDFULLY {H}{nEEDFULLY}
16 Infernal leader of hoodlums restrained by chief, shot in the arm (8) CHTHONIC {CH}{T{Ho...s}ONIC}
17 Straightforward copper probing violent cartel (5-3) CLEAR-CUT {CLEAR{CU}T*}
20 Romped in Portugal with hot European lady (6) PLAYED {P}{E+LADY}*
22 Revolting scoundrel grabs son on base - it's shocking (5) TASER {RAT<=} over {S}{E}
Reference List
Master = M, Model = T, Republican = R, Institute = I, Union = U, Fellow = F, Ecstasy = E, About = A, Caught = C, Cocaine = C, Hydrogen = H, Nitrogen = N, Chief = CH, Portugal = P, European = E, Son = S, Base = E