Runner starts to sprint, keeping improving (3)
I believe the answer is:
ski
'runner' is the definition.
(ski is a kind of runner)
'starts to sprint keeping improving' is the wordplay.
'starts to' indicates taking the first letters.
The initial letters of 'sprint keeping improving' is 'SKI'.
(Other definitions for ski that I've seen before include "Runner for use on snow" , "Walk on snow" , "run on 26 [SNOW]" , "What one might do in Alps" , "Slide on blades" .)