Be a sore winner (5)
I believe the answer is:
gloat
'be a sore winner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gloat that I've seen before include "Express triumphant self-satisfaction" , "Exult over one's good fortune" , "Take malicious satisfaction" , "Look like the cat who got the cream" , "Show malicious smugness" .)