Solution to 16D 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.
ACROSS
6 Story about indian antelope (5) SAIGA {SA{I}GA}
7 Lincoln’s last letter, “want to revisit this place in Scotland...” (8) ABERDEEN {ABE}{l...eR}{NEED<=}
10 ... really not shown as such (7) DENOTED {DE{NOT}ED} really=inDEED
11 Perhaps I can do more initially, being on the move (7) NOMADIC {I+CAN+DO+More}*
13 A day to get support, back in service (7) ANGELUS {A{{SU{LEG}N<=}
19 Search one after one for a remedy (7) PANACEA {PAN}{ACE}{A}
21 Figure regular voice will add something extra (7) OCTOGON {vOiCe}{TO GO ON} (Addendum OCTAGON {vOiCe}{TAG ON} - See comments
23 Stray sign to undress and wear dress (7) DIGRESS {D{sIGn}RESS}
25 Melt from bottom to top, fill with butter and check (7) TRAMMEL {(+t)T{RAM}MEL(-t)}
26 Given to one who is adrift at sea (5,3) FIRST AID* &lit
27 House work — choice comes later (5) MANOR {MAN}{OR}
DOWN
1 Musical on superficial affection (4,4) LION KING {LI{ON}KING}
3 Able to light up with a support — they’ll throw some light (10) CANDELABRA {CAN}{LED<=}{A}{BRA}
5 Botanist I held close left finally (6) MENDEL {M{END}E}{L}
9 Part of London remains with digital currency (1-4) E-CASH {EC}{ASH}
13 It’s for FDA, that is, to review — to be taken in as a food flovouring (10) ASAFOETIDA {FDA+IE+TO}* in {AS}{A}
15 Servants can’t open the doors, is what you hear (7) LACKEYS (~lack keys)
16 Soldier and I let out of the unit (8) R?G?M?N? (Addendum - REGIMENT {RE{GI}{ME}NT} - See comments}
20 No drink? Force in a small amount (6) NOGGIN {NO}{G}{GIN}
22 Removed from Sanatorium, so in terrible shock (6) TRAUMA {sAnAToRiUM}*
24 Criticise girls from South (4) SLAG<=
Reference List
Indian = I, Lincoln = ABE, Support = BRA, Woman = W, Left = L, Hell = DIS, Soldier = RE(Royal Engineers), Force = G(Gravity)