Many a politician is a bit wet (4)
I believe the answer is:
damp
'a bit wet' is the definition.
(damp can mean wet or moist)
'many a politician' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'politician' becomes 'MP' (Member of Parliament).
'd'+'a'+'mp'='DAMP'
'is' acts as a link.
(Other definitions for damp that I've seen before include "Somewhat moist" , "A bit wet" , "Rising -, TV sitcom" , "Humid" , "(Slightly) wet" .)