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