Food item fly has left momentarily (4)
I believe the answer is:
brie
'food item' is the definition.
(type of cheese)
'fly has left momentarily' is the wordplay.
'has left' is a deletion indicator.
'momentarily' becomes 'briefly' (synonyms).
'briefly' with 'fly' taken out is 'BRIE'.
(Other definitions for brie that I've seen before include "French cream cheese" , "cheese-producing region" , "Soft, creamy French cheese" , "Variety of cheese" , "cheesy stuff" .)