Two coppers join game (3,3)
I believe the answer is:
cup tie
'game' is the definition.
(cup tie is a kind of game)
'two coppers join' is the wordplay.
'two coppers' can mean two replacements for 'copper'.
'copper' becomes 'cu' (chemical symbol).
'copper' becomes 'p'.
'join' becomes 'tie' (I've seen this before).
'cu'+'p'+'tie'='CUP-TIE'
(Other definitions for cup tie that I've seen before include "Sports match" , "bit of competition" , "Knockout fixture in a competition" , "Knockout match" , "Sporting game" .)