Fruit turns up in an omelette (5)
I believe the answer is:
lemon
'fruit' is the definition.
(lemon is a kind of fruit)
'turns up in an omelette' is the wordplay.
'turns up' says the letters should be written backwards.
'in' indicates a hidden word.
'nomel' can be found hidden inside 'an omelette'.
'nomel' back-to-front is 'LEMON'.
(Other definitions for lemon that I've seen before include "Fruit - as an answer it's unsatisfactory" , "Leaving body" , "definitely not a peach" , "Acidic fruit" , "Sharp-tasting fruit" .)