He controls runners in winter sports (5)
I believe the answer is:
skier
'he controls runners in winter sports' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for skier that I've seen before include "sporting figure" , "Winter sportsman" , "High ball; Alpine sportsman" , "Mountain racer" , "He glides over snow, perhaps in sport" .)