Wave after couple's decisive match (10)
I believe the answer is:
tiebreaker
'decisive match' is the definition.
(a tiebreaker decides a winner)
'wave after couple's' is the wordplay.
I cannot really understand how this works, but
'wave' could be 'breaker' (breaker is a kind of wave) and 'breaker' is present in the answer.
'couple' could be 'tie' (both can mean to join) and 'tie' is found in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for tiebreaker that I've seen before include "Game decider" , "settler" , "It's decisive" , "A means of separating equal winners" .)