Game of bridge (7)

I believe the answer is:

pontoon

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But I can't explain it at all! Could you help me understand?

(Other definitions for pontoon that I've seen before include "Floating bridge support" , "Half of 1D [GAME OR BIRD]" , "variety of bridge" , "Gambling game" , "It spans" .)

I've seen this clue in The Mirror and the Evening Standard.

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