Plummy, and said to be a bit sluggish. (4)
I believe the answer is:
sloe
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for sloe that I've seen before include "Sour blackthorn fruit" , "Wild plum" , "Thorny bush with plum-like fruit" , "Fruit steeped in gin to make a liqueur" , "Black fruit" .)