Slightly damp duck in thin fog (5)
I believe the answer is:
moist
'slightly damp' is the definition.
(I've seen this before)
'duck in thin fog' is the wordplay.
'duck' becomes 'o'.
'in' is an insertion indicator.
'thin fog' becomes 'mist' (I've seen this before).
'o' put into 'mist' is 'MOIST'.
(Other definitions for moist that I've seen before include "Slightly wet, damp" , "lubricated" , "Damp or clammy" , "Damp, humid" , "Dampish" .)