Won by having lost the opposition (6)

I believe the answer is:

gained

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'won' is the definition.
('gain' can be a synonym of 'win')

I don't understand how the rest of the clue works.

(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" .)

I've seen this clue in the Evening Standard.

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