Supporters voted, as a Liberal stuffed by Conservative (9)
I believe the answer is:
advocates
'supporters' is the definition.
(people who advocate for or support someone else)
'voted as a liberal stuffed by conservative' is the wordplay.
'liberal' indicates anagramming the letters.
'stuffed by' means one lot of letters goes inside another (stuff can mean to fill or insert into).
'conservative' becomes 'C' (UK political abbreviation).
'voted'+'as'+'a'='votedasa'
'votedasa' with letters rearranged gives 'advoates'.
'advoates' going around 'c' is 'ADVOCATES'.
(Other definitions for advocates that I've seen before include "briefs?" , "Recommends" .)