Fellow ate wrong dairy product (4)
I believe the answer is:
feta
'dairy product' is the definition.
(type of cheese)
'fellow ate wrong' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'wrong' is an anagram indicator.
'ate' anagrammed gives 'eta'.
'f'+'eta'='FETA'
(Other definitions for feta that I've seen before include "White Greek cheese" , "Salty goat or sheep milk cheese" , "Eastern Mediterranean cheese" , "A ewe's, goat's milk cheese" , "Soft Greek cheese" .)