Good marriage is an easily-won game (4,5)
I believe the answer is:
love match
'good marriage' is the definition.
'love match' can be an answer for 'marriage' (love match is a kind of marriage). I'm not sure about the 'good' bit.
'an easily-won game' is the wordplay.
'an easily-won' becomes 'love' (I can't explain this - if you can you should believe this answer much more).
'game' becomes 'match' (eg a football match).
'love'+'match'='LOVE-MATCH'
'is' is the link.
Can you help me to learn more?
(Other definitions for love match that I've seen before include "comes to nothing for one party [IN?] court?" , "Marriage based on real affection" , "passionate union meeting" , "There's nothing to equal" , "Basis of successful partnership" .)