Diamonds to pawn, backing the old game (3,6)
I believe the answer is:
ice hockey
'game' is the definition.
(ice hockey is a kind of game)
'diamonds to pawn backing the old' is the wordplay.
'diamonds' becomes 'ice' (slang term for diamonds).
'to pawn' becomes 'hock' (hock can mean to pawn an item at a pawnbroker's).
'backing' says the letters should be written in reverse.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'ye' backwards is 'ey'.
'ice'+'hock'+'ey'='ICE-HOCKEY'
(Other definitions for ice hockey that I've seen before include "Team game played with long sticks" , "Violent sport, big in North America" , "Sport with puck" , "Game played by skaters with a puck" .)