Runner starts first on King Island (3)
I believe the answer is:
ski
'runner' is the definition.
(something which 'runs', ie moves quickly on snow)
'starts first on king island' is the wordplay.
'first' says to take the initial letters.
'on' means one lot of letters go next to another.
'king' becomes 'k' (abbreviation for king in chess).
'island' becomes 'i'.
The initial letter of 'starts' is 's'.
's'+'k'+'i'='SKI'
(Other definitions for ski that I've seen before include "the end for many Russians" , "Move over snow or water" , "Walk on snow" , "Winter-sports item" , "Slide on blades" .)