Be a poor winner (5)
I believe the answer is:
gloat
'be a poor 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 "Dwell on with self-satisfaction" , "Show malicious smugness" , "Look triumphant" , "Look like the cat who got the cream" , "Smugly look on another's misfortune" .)