Fellow finds new diet disgusting (5)
I believe the answer is:
fetid
'disgusting' is the definition.
(thesaurus)
'fellow finds new diet' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'finds' says to put letters next to each other.
'new' is an anagram indicator.
'diet' is an anagram of 'etid'.
'f'+'etid'='FETID'
(Other definitions for fetid that I've seen before include "Stinking, perhaps breath" , "Malodorous, stinking" , "Smelling bad, stinking" , "Putrid and stinking" , "Noxious" .)