Be a bad winner (5)
I believe the answer is:
gloat
'be a bad winner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gloat that I've seen before include "Smugly look on another's misfortune" , "Exult maliciously" , "Take malicious satisfaction" , "Be smug about one's success" , "Express triumphant self-satisfaction" .)