Runner’s second kilometre, impressive at first (3)
I believe the answer is:
ski
'runner's' is the definition.
(something which 'runs', ie moves quickly on snow)
'second kilometre impressive at first' is the wordplay.
'at first' says to take the initial letters.
The first letters of 'second kilometre impressive' is 'SKI'.
(Other definitions for ski that I've seen before include "Winter sport item" , "Use runners crossing snow" , "Narrow runner built for travel in snow" , "Pole? How one often ends" , "Go downhill rapidly" .)