Dull Conservative retains seat (7)
I believe the answer is:
coarsen
'dull' is the definition.
(I've seen this before)
'conservative retains seat' is the wordplay.
'conservative' becomes 'con' (abbreviation for Conservative).
'retains' indicates putting letters inside.
'seat' becomes 'arse' (synonyms).
'con' placed around 'arse' is 'COARSEN'.
(Other definitions for coarsen that I've seen before include "become inferior" , "Make rougher" , "Make rough or crude" , "Turn rough" , "Dull" .)