Conservative restrained by a simple fine (6)
I believe the answer is:
amerce
'fine' is the definition.
(I know that amerce is a more specific form of the action fine)
'conservative restrained by a simple' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'restrained by' indicates putting letters inside.
'simple' becomes 'mere' (synonyms).
'a'+'mere'='amere'
'c' going within 'amere' is 'AMERCE'.
(Other definitions for amerce that I've seen before include "Came re the fine" , "Punish by imposing arbitrary fine" .)