Forbid a rigged vote (4)
I believe the answer is:
veto
'forbid' is the definition.
(I know that forbid can be written as veto)
'a rigged vote' is the wordplay.
'a rigged' indicates anagramming the letters.
'vote' with letters rearranged gives 'VETO'.
(Other definitions for veto that I've seen before include "A confused vote, altogether negative" , "Refuse to authorise" , "Right to reject a decision" , "Don't allow" , "Right to prevent proposed action" .)