Won by having lost the opposition (6)
I believe the answer is:
gained
'won' is the definition.
('gain' can be a synonym of 'win')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gained that I've seen before include "Attained, earned" , "Made a profit, acquired" , "Earned or increased" , "Made profit or made progress" , "Augmented or expanded" .)