Plump for Conservative partner (6)
I believe the answer is:
chubby
'plump' is the definition.
(synonyms)
'conservative partner' is the wordplay.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'partner' becomes 'hubby' (informal term for husband).
'c'+'hubby'='CHUBBY'
'for' acts as a link.
(Other definitions for chubby that I've seen before include "Podgy" , "On the plump side" , "Round and pleasantly plump" , "Attractively plump" , "Fat in the face" .)