Fast runner down mountains (3)
I believe the answer is:
ski
'fast runner' is the definition.
'ski' can be an answer for 'runner' (something which 'runs', ie moves quickly on snow). I'm unsure of the 'fast' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ski that I've seen before include "Narrow runner for snow and water" , "Piece of sports equipment" , "Winter sports footwear" , "Winter sport attachment" , "Mountain runner" .)