Fellow finds new diet smelly (5)
I believe the answer is:
fetid
'smelly' is the definition.
('fetid' can be a synonym of 'smelly')
'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' indicates anagramming the letters.
'diet' anagrammed gives 'etid'.
'f'+'etid'='FETID'
(Other definitions for fetid that I've seen before include "Foul-smelling or stinking" , "Noisome" , "Smelling bad, stinking" , "Stinking, perhaps breath" , "Having a stale and unpleasant smell" .)