Fruit barrel turned upside-down (3)
I believe the answer is:
nut
'fruit' is the definition.
(nut is a kind of fruit)
'barrel turned upside-down' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'turned upside down' is a reversal indicator.
'tun' back-to-front is 'NUT'.
(Other definitions for nut that I've seen before include "Enthusiastic person" , "Fan" , "buff" , "Bolt's partner" , "eg Pecan" .)