Runner quietly failing to jump (3)
I believe the answer is:
ski
'runner' is the definition.
(something which 'runs', ie moves quickly on snow)
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 "Winter sport equipment" , "Slide on snow" , "Biathlon runner" , "Narrow runner built for travel in snow" , "runner in winter" .)