Veto for proposal (6)
I believe the answer is:
forbid
'veto' is the definition.
(I know that veto can be written as forbid)
'for proposal' is the wordplay.
'proposal' becomes 'bid' (a bid is a proposal or offer).
'for'+'bid'='FORBID'
(Other definitions for forbid that I've seen before include "Outlaw" , "Refuse permission, do not allow" , "Deny, prohibit" , "Refuse permission, order not to" , "Interdict" .)