Blackthorn (4)
I believe the answer is:
sloe
'blackthorn' is the definition.
(synonyms)
This is all the clue.
(Other definitions for sloe that I've seen before include "Blackthorn, used in making a type of gin" , "Fruit often used for gin" , "Musical study" , "Black fruit" , "Thorny bush with plum-like fruit" .)