Pressure after barrier is wet (4)
I believe the answer is:
damp
'is wet' is the definition.
('damp' can be similar in meaning to 'wet')
'pressure after barrier' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'after' means one lot of letters go next to another.
'barrier' becomes 'dam' (dam is a kind of barrier).
'p' put after 'dam' is 'DAMP'.
(Other definitions for damp that I've seen before include "Slightly moist" , "Unenthusiastic" , "(Slightly) wet" , "Moisture" , "soggy" .)