Solution to 8D 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 solve, if not solved by 1 PM.
ACROSS
1 Plug of TV set remained loose (13) ADVERTISEMENT*
8 Surprises mother, finally cuts long hair (5) ROCKS {m...eR}{lOCKS}
9 & 2D Seem promising but ultimately disappoint with tale for detective novel (7,2,7) FLATTER TO DECEIVE {TALE+FOR+DETECTIVE}*
11 Raw food consumed in a puerile manner (10) CHILDISHLY {CHIL{DISH}LY}
12 Lounge in flames! Save boy (4) LAZE bLAZE
14 Act foolishly, repent getting drunk at premiere (7) PRETEND {REPENT*}{Dr..k}
16 Sore about husband cuddling secretary, start to take brave course of action (7) WARPATH {RAW<=}{H} over {PA}{Take}
19 Party hosted by gal on fancy boat (7) GONDOLA {DO} in {GAL+ON}*
21 Parched? Relief to drink water finally (4) ARID {A{w..eR}ID}
22 Fearless guards taunt criminals at centre to engage in a decisive gunfight (5,2,3) SHOOT IT OUT {STOUT} over {HOOT{c..mIn..s}
25 Enthralled by demure American on trip (9) ENAMOURED {DEMURE+A+ON}*
26 Stopped in English retro joint to have a little drink (5) ENDED {E}{DE{Dr..k}N<=}
27 What welder might do to arouse sudden anger (4,6,3) MAKE SPARKS FLY [DD]
DOWN
2 See 9 Across
3 A seedy sot's going berserk around island - be careful! (4,4,2) EASY DOES IT {A+SEEDY+SOT}* over {I}
4 Revolting drunkard tackling couple of fine upper-class people (5) TOFFS {SOT<=} over {FF}
6 Fantasy of steamy threesome (4) MYTH [T]
7 Northern rogue in a rave, delivering ultimately pure ecstasy (7) NIRVANA {N}{IN+A+RAVe}*
8 Match price to secure revolutionary diamonds for top collector (11) R?C?P?O?A?E ()
10 Generous doctor! He operated without remuneration finally (4-7) OPEN-HEARTED {HE OPERATED}* over {r...oN}
13 Popularity of new dinner sets (10) TRENDINESS*
15 The rat scurried into apartment over unsafe structure (9) DEATHTRAP {THE+RAT}* in {PAD<=}
18 Folk tale character acting upset about protecting immature girl (3,4) ALI BABA {A}{LI{BABe}A<=}
20 Egg-shaped cavity in gemstone, not soft (7) OVOIDAL {Op{VOID}AL}
23 Renovate retreats surrounding lake, becoming more decrepit (5) OLDER {RED{L}O<=}
24 Wish hot guy removes top (4) HOPE {H}{rOPE}
Reference List
Long = L, Boy = B, Husband = H, Over = O, Party = DO, American = A, English = E, Island = I, Fine = F, Northern = N, Acting = A, Soft = P, Lake = L, Hot = H