Sour plum (4)
I believe the answer is:
sloe
'sour plum' 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 "Sour blackthorn fruit" , "Fruit of the blackthorn does't sound fast" , "Wild plum" , "Black fruit" , "Fruit steeped in gin to make a liqueur" .)