In favour of retreat after offer is vetoed (9)
I believe the answer is:
forbidden
'is vetoed' is the definition.
(thesaurus)
'in favour of retreat after offer' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'retreat' becomes 'den'.
'after' means one lot of letters go next to another.
'offer' becomes 'bid' ('bid' can be a synonym of 'offer').
'den' after 'bid' is 'bidden'.
'for'+'bidden'='FORBIDDEN'
(Other definitions for forbidden that I've seen before include "This fruit is always more enticing" , "Verboten, prohibited" , "not 35 [PERMISSIBLE]" , "Illicit pleasure" , "Proscribed" .)