Marathoner or Indy driver (5)
I believe the answer is:
racer
I believe this clue is a double definition.
'marathoner' is the first definition.
(I've seen this in another clue)
'indy driver' is the second definition.
(I've seen this in another clue)
(Other definitions for racer that I've seen before include "Participant in speed contest" , "One car" , "someone driving very fast?" , "Athlete" , "American snake" .)