Move over, easily demolishing lead of marathon runner (3)
I believe the answer is:
ski
'runner' is the definition.
(ski is a kind of runner)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for ski that I've seen before include "Piece of sports equipment" , "Winter sport item" , "Pole? How one often ends" , "Engage in a winter sport" , "Go downhill rapidly" .)