Jock (7)
I believe the answer is:
athlete
'jock' is the definition.
(I know that jock can be written as athlete)
This is all the clue.
(Other definitions for athlete that I've seen before include "E.g. runner or jumper" , "gymnast, maybe" , "Track and field competitor" , "Competitive runner, e.g." , "One trains in sport" .)