Middle-distance runner (5)
I believe the answer is:
miler
'middle-distance runner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for miler that I've seen before include "Athlete running 1,760 yards" , "Runner over a specific distance" , "Pre-metric middle-distance runner" , "horse" , "one who competes in the long run" .)