Solution to 2D 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 Left enthralled by great retired star, most suave (8) GLIBBEST {BI{L}G<=}{BEST}
5 Revealed love to woman in shack (6) SHOWED {SH{O}{W}ED}
10 Champion seen tottering outside retro bar (7) ENDORSE {EN{ROD<=}SE*}
11 Wanting new cultural fair (7) ETHICAL ETHnICAL
12 Aristocrat welcoming a British billionaire (5) NABOB {N{A}{B}OB}
13 Desiring hot girl in T-shirt (9) THIRSTING {G+IN+T+SHIRT}*
14 Facing a difficult situation at home with a sozzled son on cannabis (2,1,5,4) IN A TIGHTSPOT {IN}{A}{TIGHT}{S}{POT}
18 Very small amount of hard cash found in a tattered shirt (5-7) HAIRS-BREADTH {H} and {BREAD} in {A+SHIRT}*
21 Most important law announced (9) PRINCIPAL (~principle)
23 Screw in beam is eroded (5) MISER [T]
24 One who helps in a venture on hill (7) ABETTOR {A}{BET}{TOR}
25 Ecstatic with grand success, breaking record (7) GLOWING {G}{LO{WIN}G}
26 Stud picking up model is Polish officer (6) HETMAN {HE{T}MAN}
27 Dear me! Worried after street gets flooded (8) STREAMED {DEAR+ME}*<=>{ST}
DOWN
2 Young socialite entering popular bar is owing money (2,4) I?D?B? (Addendum - IN DEBT {IN}{DEB}{T} - See comments)
3 Most basic details of babe cavorting with Senhor when husband's away (4,5) BARE BONES {BABE+SENhOR}*
4 Top actress performed before opening game watched by many (9,5) SPECTATOR SPORT {TOP+ACTRESS}*{PORT}
6 Associate enthralled by hash dances and laughs (2-3) HA-HAS {A} in {HASH}*
7 Native American huts in hamlet on island in a higher position on top of slope (8) WICKIUPS {WICK}{I}{UP}{Sl..e}
9 Understanding result of opening curtain in the morning (6,8) SEEING DAYLIGHT [DD]
15 Tribal emblem and mysterious tome found in abandoned plot on base (5,4) TOTEM POLE {TOME}* in {PLOT}* on {E}
16 Start to help after wife slipped - has lip and neck injury (8) WHIPLASH {Help}<=>{W} and {HAS+LIP}*
17 Earnest model dieting after gaining pounds (8) DILIGENT {DI{L}IGENT*}
20 Hard after fellow becomes passionless (6) FRIGID {RIGID}<=>{F}
Reference List
Left = L, Woman = W, New = N, British = B, Girl = G, Son = S, Hard = H, Grand = G, Model = T, Popular = IN, Bar = T, Husband = H, Island = I, European = E, Base = E, Wife = W, Pounds = L, Fellow = F, Catholic = C