Track athlete (6)
I believe the answer is:
runner
'track athlete' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for runner that I've seen before include "Horse in a race; shoot" , "Kind of bean is a fast mover" , "Branch from the base of plant that produces new plants" , "Marathon athlete?" , "-- bean, vegetable" .)