Runner outside unable to finish (3)
I believe the answer is:
ski
'runner' is the definition.
(something which 'runs', ie moves quickly on snow)
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 "the end for many Russians" , "Glide or race across snow with feet attachments" , "Glide ove snow" , "Move over snow or water" , "Piece of sports equipment" .)