Run-off that is in front of the wave (10)
I believe the answer is:
'run-off' is the definition.
I can't tell whether this defines the answer.
'that is in front of the wave' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'in front of' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'wave' becomes 'breaker' (breaker is a kind of wave).
'ie' after 't' is 'tie'.
Can you help me to learn more?
(Other definitions for tiebreaker that I've seen before include "It's decisive" , "A means of separating equal winners" , "Game decider" , "settler" .)