A politician in retreat's wet (6)
I believe the answer is:
dampen
'wet' is the definition.
(dampening is a kind of wetting)
'a politician in retreat's' is the wordplay.
'politician' becomes 'mp'.
'in' means one lot of letters goes inside another.
'retreat' becomes 'den'.
'a'+'mp'='amp'
'amp' put inside 'den' is 'DAMPEN'.
(Other definitions for dampen that I've seen before include "Make slightly moist, or discourage" , "Make moist (6)" , "Discourage in moist manner" , "Make moist or depressed" , "Apply liquid!" .)