Barrier and enclosure wet (6)
I believe the answer is:
dampen
'wet' is the definition.
(dampening is a kind of wetting)
'barrier and enclosure' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'and' is a charade indicator (letters next to each other).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'dam'+'pen'='DAMPEN'
(Other definitions for dampen that I've seen before include "Make moist or depress spirit" , "Discourage in moist manner" , "get a bit wet" , "Make moist or subdued" , "Moisten" .)