Athlete's agent (6)
I believe the answer is:
runner
This clue is a double definition.
'athlete's' is the first definition.
(runner is a kind of athlete)
'agent' is the second definition.
(thesaurus)
(Other definitions for runner that I've seen before include "Type of bean for an athlete?" , "Kind of bean for an athlete" , "Groove or strip on which a drawer slides" , "Messenger; plant shoot" , "Long narrow carpet" .)