One asserting he will play the dummy (8)
I believe the answer is:
declarer
This clue is a double definition.
'one asserting' is the first definition.
(someone asserting or declaring things)
'he will play the dummy' is the second definition.
(the declarer plays the dummy in bridge)
(Other definitions for declarer that I've seen before include "One tries to make contract" , "Bridge player choosing trumps" , "Successful bidder (bridge)" .)