Mother swan’s wet (6)
I believe the answer is:
dampen
'wet' is the definition.
(dampening is a kind of wetting)
'mother swan's' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'swan' becomes 'pen' (pen is a kind of swan).
'dam'+'pen'='DAMPEN'
(Other definitions for dampen that I've seen before include "Make moist (6)" , "Make moist or depress spirit" , "Make moist or depress spirits" , "Make slightly moist, or discourage" , "Moisten" .)