Solution to 21D 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 One who observes husband tottering, overwhelmed by drink (8) BEHOLDER {BE{H}{OLD}ER}
5 Prodigy beginning to work on developing drone (6) WONDER {Work}{DRONE}*
10 View small insect going around lake (5) SLANT {S}{L}{ANT}
11 Fixed mindset is leading to diffidence (9) TIMIDNESS*
12 Without any limitations, like loafers? (2,7,8) (Continued in 17 Down) NO STRINGS ATTACHED [C&DD]
13 Wants fine food and more rum (5) ODDER fODDER
15 Satisfy upper-class female with fine diamonds after a little spat (7) SUFFICE {U}{F}{F}{ICE}<=>{Spat}
18 Little people desire to cut cake in retreat (7) TITCHES {ITCH} in {SET<=}
20 Envies model surrounded by men (6) COVETS {COVE{T}S}
24 Possibly bit nervous going topless? That's obvious (9) OBTRUSIVE {BIT+nERVOUS}*
25 Clumsy and daft, he is beginning to make blunders (3-6) HAM-FISTED {DAFT+HE+IS+Make}*
26 Glide over snow around an Asian city (5) IKSAN {SKI<=}{AN}
27 Model is turning on girl in retreat (6) DESIGN {DE{IS<=}{G}N}
28 Homeless feline lately wandering around area behind college (5,3) ALLEY CAT {LATELY}* over {A}<=>{C}
DOWN
1 Mounting gossip about immorality in building (6) BASING {GAB<=} over {SIN}
2 Democrat leading in preliminary contest getting early advantage (4,5) HEAD START {D}{STAR} in {HEAT}
3 Creative way of solving problems after a fine leader wins polls at centre (7,8) LATERAL THINKING {LATER}{A}{THIN}{KING} over {poLls}
4 President entering to accept treaty (7) ENTENTE [T]
6 Weird nuts in USA vote for likely winner (4-2,9) ODDS-ON FAVOURITE {ODD}{IN+USA+VOTE+FOR}*
7 Fear's gone after taking therapy essentially (5) DREAD {D{theRapy}EAD}
8 Cool being engaged (8) RESERVED [DD]
16 Perhaps, ricin isn't genuine (9) INTRINSIC*
17 See 12 Across
19 Small bananas becoming mottled (6) SPOTTY {S}{POTTY}
20 Scoundrel having sex with English lady in castle (7) CITADEL {C{IT}AD}{E}{L}
Reference List
Husband = H, Small = S, Lake = L, Fine = F, Upper class = U, Female = F, Diamonds = ICE, Model = T, Girl = G, Area = A, College = C, Democrat = D, Sex = IT, English = E, Lady = L, Posh = U, Bar = T