Fellow finds new diet offensive (5)
I believe the answer is:
fetid
'offensive' is the definition.
(thesaurus)
'fellow finds new diet' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'finds' is a charade indicator (letters next to each other).
'new' indicates anagramming the letters.
'diet' anagrammed gives 'etid'.
'f'+'etid'='FETID'
(Other definitions for fetid that I've seen before include "Smelling bad, stinking" , "Stinking, like animal breath" , "Noxious" , "Foul-smelling or stinking" , "Putrid and stinking" .)