Party member in protest about informer (8)
I believe the answer is:
democrat
'party member' is the definition.
(member of the Democrat Party)
'protest about informer' is the wordplay.
'protest' becomes 'demo' (demonstration).
'about' becomes 'c' (circa).
'informer' becomes 'rat' (synonyms).
'demo'+'c'+'rat'='DEMOCRAT'
'in' is the link.
(Other definitions for democrat that I've seen before include "Believer in popular government" , "Member of one of US parties" , "Cleveland?" , "US politician" , "Political type" .)