Runner's expertise when avoiding lines (3)
I believe the answer is:
'runner's' is the definition.
(something which 'runs', ie moves quickly on snow)
'expertise when avoiding lines' is the wordplay.
'expertise' becomes 'skill' (I've seen this in another clue).
'when avoiding' is a deletion indicator.
'lines' becomes 'll' (an abbreviation for line written twice).
'skill' with 'll' taken away is 'SKI'.
(Other definitions for ski that I've seen before include "Slide downhill, sportingly" , "Pole? How one often ends" , "Biathlon runner" , "Runner for use on snow" , "Winter sports equipment" .)