Exist as a dissenter in the party (5)
I believe the answer is:
beano
'party' is the definition.
(I've seen this before)
'exist as a dissenter' is the wordplay.
'exist as' becomes 'be'.
'dissenter' becomes 'no' (someone opposing something might be a 'no').
'be'+'a'+'no'='BEANO'
'in the' acts as a link.
(Other definitions for beano that I've seen before include "Junket - children's comic" , "Comic - party" , "Comic and informal get-together" , "Informal party" , "Enjoyable party" .)