Tart fruit (4)
I believe the answer is:
sloe
'tart fruit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sloe that I've seen before include "Musical study" , "Fruit of the blackthorn does't sound fast" , "Black fruit" , "Blackthorn, used in making a type of gin" , "Thorny bush with plum-like fruit" .)