Solution to 7D 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.
Any novice to identify the theme and the Nina.
ACROSS
9 Famous setter seen every now and then in retreat in La Sierra (a club, abroad) (9) ARAUCARIA {lA+sIeRrA+a+ClUb+AbRoAd}<=
12 Tree with unusual floral activity finally replaced by kiss in Hindi cinema (7) BOXWOOD {BO(-lly+x)XWOOD}
14 Appalled, I scorn appropriating hip-hop for some junk metal (5,4) SCRAP IRON {I+SCORN}* over {RAP}
16 Sticky substance (a polymer) starts to thicken, unleashing inflammation and warmth (4-11) GOOD-NATUREDNESS {GOO}{DNA}{Th...n}{Un...g}{REDNESS}
19 Extremely crude bots redesigned to incorporate sight (9) OBSCENEST {OB{SCENE}ST*}
22 Family members least nuanced about hugging? (5) AUNTS [T<=]
26 Peacekeepers getting terribly bored took off clothes (7) UNROBED {UN}{BORED*}
27 Even unfinished and sloppy lacquer gets rid of the onset of corrosion (5) EQUAL LAcQUEr*
28 In the morning, they might offer guidance for show; after a bit of breakfast, they are surprisingly offensive! (9) AMBUSHERS {AM} and {USHERS}<=>{Br...t}
DOWN
1 Turning a long way, endlessly left — playing for draw? (6) RAFFLE {FAR<=}{LEFt*}
2 Sure about acquiring flamboyant angora jumper? (8) KANGAROO {OK<=} over {ANGORA}*
4 Bradman accumulates runs with England’s seaming openers and goes on and on (6) DRONES {D{R}ON}{En...d}{Se...g}
5 Pitcher full of bad, spoiled beer gets one mouthing off (8) JABBERER {JA{B}{BEER*}R}
6 “Nap/sex?” — partners going for tip (4) APEX {nAP+sEX}
7 Appreciate one avoiding unnecessary expenditure for auditors (6) ?A?O?R (Addendum - SAVOUR (~ saver) - See comments)
8 Anyone suffering after swallowing some dubiously “special” painkillers? (8) ANODYNES {ANO{Du...y}YNE*}{S}
15 Doctor’s regularly used app, or compiler of detailed records over time (10) PODIATRIST {aPp+Or}{DIA{T}RIST}
16 George Eliot was a girl? This further confused Leelawati so badly (8) GROGGIER {GROGGIER+LEELAWATI=GEORGE+ELIOT+WAS+A+GIRL} [CA]
17 After putting away all jackets, why must butler say archly, “What you normally order?” (3,5) THE USUAL {wHy+mUSt+bUTLEr+sAy}*
18 Dignified short hat lying around, oddly rejected already (8) ENNOBLED {BONNEt<=}{aLrEaDy}
20 Purchase pans (4,2) SNAP UP {PANS}* [RA]
21 These may go up and down during an election? (6) THUMBS [CD]
23 Secretly admires a distinctly evil person (6) SADIST [T]
25 Bless Bertie’s bloody partner! (4) ALLY bALLY
Reference List
Rook = R, Kiss = X, Husband = H, About = C, My = COR, Runs = R, Bad = B, Partners = N,s {north and South in Bridge card game), Special = S, Time = T, Bertie = B