Vote against party (5)
I believe the answer is:
beano
'party' is the definition.
(I've seen this before)
'vote against' is the wordplay.
'vote' becomes 'bea' (I can't explain this - if you can you should give a lot more credence to this answer).
'against' becomes 'no'.
'bea'+'no'='BEANO'
(Other definitions for beano that I've seen before include "A bone for a feast" , "Celebration" , "Function" , "Comic - party" , "Comic and informal get-together" .)