Protest about informer being party member (8)
I believe the answer is:
democrat
'party member' is the definition.
(thesaurus)
'protest about informer' is the wordplay.
'protest' becomes 'demo' (I've seen this before).
'about' becomes 'c' (abbreviation for 'circa'**).
'informer' becomes 'rat' ('rat' can be a synonym of 'informer').
'demo'+'c'+'rat'='DEMOCRAT'
'being' acts as a link.
(Other definitions for democrat that I've seen before include "One who believes in government by the whole people" , "Cleveland?" , "US political party supporter" , "Advocate of government by elected representatives" , "Believer in government of, for and by the people" .)