Runner's expertise when avoiding lines (3)
I believe the answer is:
ski
'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" .)