Unpleasantly moist (4)
I believe the answer is:
dank
'unpleasantly moist' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dank that I've seen before include "(Of atmosphere) wet and unpleasant" , "Chilly and clammy" , "Unpleasantly cool and damp" , "Unpleasantly cold and humid" , "Cold and moist" .)