If you start this event, you will never reach the end (5,4)
I believe the answer is:
relay race
'if you start this event you will never reach the end' is the definition.
'relay race' can be an answer for 'event' (relay race is a kind of event). I am not sure about the rest of the definition.
'if you start this event' is the wordplay.
I cannot really see how this works, but
'event' could be 'relay' (I've seen this before) and 'relay' is located in the answer.
The remaining letters 'race' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for relay race that I've seen before include "Foot speed contest passing batons" , "having started, one couldn't finish it" , "Speed contest with teams of four runners" , "Team contest" , "Event in which a baton is passed" .)