Game you would be mad to lose (7)

I believe the answer is:

marbles

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'game' is the definition.
(marbles is a kind of game)

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

(Other definitions for marbles that I've seen before include "You'll be mad if you lose them" , "alleys, perhaps" , "Small spheres of glass - statuary" , "Glass balls to play with" , "Children's game played with little balls" .)

I've seen this clue in The Mirror.

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