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