Deny being in favour of proposal (6)
I believe the answer is:
forbid
'deny' is the definition.
(thesaurus)
'in favour of proposal' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'proposal' becomes 'bid' (I've seen this in other clues).
'for'+'bid'='FORBID'
'being' is the link.
(Other definitions for forbid that I've seen before include "Outlaw" , "bar" , "Refuse permission, order not to" , "Prohibit, veto" , "Ban" .)