Card game on which bridge is based? (7)
I believe the answer is:
pontoon
I believe this clue is a double definition.
'card game' is the first definition.
(I've seen this before)
'on which bridge is based?' is the second definition.
(a bridge may be supported by pontoons)
(Other definitions for pontoon that I've seen before include "Gambling game" , "It spans" , "Vingt-et-un" , "Floating bridge support" , "Half of 1D [GAME OR BIRD]" .)