Runner in early event not starting back (5)
I believe the answer is:
miler
'runner' is the definition.
(miler is a kind of runner)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for miler that I've seen before include "Athlete running 1,760 yards" , "horse" , "Pre-metric middle-distance runner" , "Runner over a specific distance" , "one who competes in the long run" .)