The champion, dismissing bridge team as less substantial (7)
I believe the answer is:
thinner
'the champion' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
I don't understand how the remainder of the clue works.
(Other definitions for thinner that I've seen before include "Slenderer" , "Diluting agent" , "what one adds to paint" , "Skinnier" , "Less fat" .)