Protest about blackleg, a party member (8)
I believe the answer is:
democrat
'a party member' is the definition.
(thesaurus)
'protest about blackleg' is the wordplay.
'protest' becomes 'demo' (I've seen this before).
'about' becomes 'c' (abbreviation for 'circa').
'blackleg' becomes 'rat' ('rat' can be a synonym of 'blackleg').
'demo'+'c'+'rat'='DEMOCRAT'
(Other definitions for democrat that I've seen before include "Ditto" , "Political type" , "Member of US political party" , "US politician" , "Believer in popular government" .)