A few sneaky clues I can't quite work out:-
7 Down - "Keep at it in one's absence" (4) -O-T
11 Across:- "Additional mouth sound" (4) M--E
12 Down:- "A covering letter from Greece, say" (5) --I--
22 Across:- "Give Labrador's lead back" (4) L--D
(the "D" on the end of the last one probably
signifies the last letter of "lead").