Outlaw in favour of offer (6)
I believe the answer is:
forbid
'outlaw' is the definition.
(I know that outlaw is a more specific form of the action forbid)
'in favour of offer' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'offer' becomes 'bid' (synonyms).
'for'+'bid'='FORBID'
(Other definitions for forbid that I've seen before include "Prohibit, veto" , "Refuse permission, do not allow" , "Refuse permission, order not to" , "Outlaw" , "bar" .)