Fruit, old, turning bad (5)
I believe the answer is:
olive
'fruit' is the definition.
(I've seen this before)
'old turning bad' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'turning' is a reversal indicator.
'bad' becomes 'evil' ('evil' is associated in meaning with 'bad').
'evil' backwards is 'live'.
'o'+'live'='OLIVE'
(Other definitions for olive that I've seen before include "Mediterranean fruit or greenish colour" , "Female" , "Yellowish-green colour" , "Mediterranean tree-fruit - nice in martini" , "Evergreen Mediterranean tree" .)