Slightly damp duck in fog (5)
I believe the answer is:
moist
'slightly damp' is the definition.
(I've seen this before)
'duck in fog' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'in' is an insertion indicator.
'fog' becomes 'mist' (mist is a kind of fog).
'o' going inside 'mist' is 'MOIST'.
(Other definitions for moist that I've seen before include "Slightly wet, damp" , "sloppy" , "Damp, humid" , "bit wet" , "Damp or clammy" .)