Marriage partner introducing grand game (6)
I believe the answer is:
bridge
'game' is the definition.
(bridge is a kind of game)
'marriage partner introducing grand' is the wordplay.
'marriage partner' becomes 'bride' (I am not sure about this - if you are sure you should believe this answer much more).
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'bride' enclosing 'g' is 'BRIDGE'.
Can you help me to learn more?
(Other definitions for bridge that I've seen before include "going across river?" , "Span of popular card game" , "One on the fiddle" , "played by a quartet" , "Card game; structure" .)