Score, and I will secure this game (6-3)

I believe the answer is:

twenty-one

But I can't explain it at all!

(Other definitions for twenty-one that I've seen before include "Card game, blackjack or pontoon" , "XXI - used to be the coming of age year" , "Target total in blackjack or pontoon" .)

I've seen this clue in The Times.

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