Runner starts to sprint and keeps improving (3)

I believe the answer is:

ski

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'runner' is the definition.
(something which 'runs', ie moves quickly on snow)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ski that I've seen before include "Slide on snow" , "Snow blade" , "Use runners crossing snow" , "Go downhill rapidly" , "Move over snow or water" .)

I've seen this clue in The Times.

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