Loser, full of ego, short of 50%, used to celebrate victory (9)
I believe the answer is:
triumphal
But I can't explain it at all! Could you help me understand?
(Other definitions for triumphal that I've seen before include "Exulting in success" , "Scenting victory" , "Type of arch (as in Paris)" , "Celebrating a success" , "Showing excessive exultation" .)