Fellow ate wrong cheese (4)
I believe the answer is:
feta
'cheese' is the definition.
(I've seen this before)
'fellow ate wrong' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'wrong' indicates anagramming the letters.
'ate' anagrammed gives 'eta'.
'f'+'eta'='FETA'
(Other definitions for feta that I've seen before include "Salty goat or sheep milk cheese" , "Goat's or sheep's milk cheese" , "Low-fat cheese" , "A ewe's, goat's milk cheese" , "Greek 8 across" .)