Fed no bird nuts -- it's banned (9)
I believe the answer is:
forbidden
'banned' is the definition.
(I've seen this before)
'fed no bird nuts' is the wordplay.
'nuts' indicates anagramming the letters (nuts can mean crazy).
'fed'+'no'+'bird'='fednobird'
'fednobird' anagrammed gives 'FORBIDDEN'.
'it's' acts as a link.
(Other definitions for forbidden that I've seen before include "Not allowed, banned" , "Illicit pleasure" , "Proscribed" , "Verboten, prohibited" , "Prohibited, taboo" .)