Upset no repast is without a fruit (5)
I believe the answer is:
lemon
'fruit' is the definition.
(lemon is a kind of fruit)
'upset no repast is without a' is the wordplay.
'upset' is a reversal indicator.
'repast' becomes 'meal' ('meal' can be a synonym of 'repast').
'is without' is a deletion indicator.
'no'+'meal'='nomeal'
'nomeal' with 'a' taken away is 'nomel'.
'nomel' back-to-front is 'LEMON'.
(Other definitions for lemon that I've seen before include "Pulsate" , "A fruit; a worthless item" , "Fruit; sort of grass, sole" , "Dab" , "leaving bitter taste?" .)