Start doubting a parliamentarian is wet (4)
I believe the answer is:
damp
'is wet' is the definition.
('damp' can be similar in meaning to 'wet')
'start doubting a parliamentarian' is the wordplay.
'start' indicates taking the first letters.
'parliamentarian' becomes 'mp'.
The initial letter of 'doubting' is 'd'.
'd'+'a'+'mp'='DAMP'
(Other definitions for damp that I've seen before include "soggy" , "Rising -, TV sitcom" , "Humid" , "Moisture" , "(Slightly) wet" .)