Damp fog — nothing visible in that (5)
I believe the answer is:
moist
'damp' is the definition.
(synonyms)
'fog nothing visible in that' is the wordplay.
'fog' becomes 'mist' (mist is a kind of fog).
'nothing' becomes 'O' (looks like zero - 0).
'visible in that' means one lot of letters goes inside another.
'mist' placed around 'o' is 'MOIST'.
(Other definitions for moist that I've seen before include "Damp, humid" , "Damp or clammy" , "lubricated" , "bit wet" , "Dampish" .)