Athlete who looks happy, though not first (5)
I believe the answer is:
miler
'athlete' is the definition.
(person trained to run a mile)
I can't explain the rest of the clue.
(Other definitions for miler that I've seen before include "one who competes in the long run" , "Athlete running 1,760 yards" , "horse" , "Pre-metric middle-distance runner" , "Runner over a specific distance" .)