Runner is climbing over the summit of Kilimanjaro (3)
I believe the answer is:
ski
'runner' is the definition.
(something which 'runs', ie moves quickly on snow)
'is climbing over the summit of kilimanjaro' is the wordplay.
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'over' indicates putting letters inside.
'the summit of' indicates taking the first letters.
The initial letter of 'kilimanjaro' is 'k'.
'is' back-to-front is 'si'.
'si' placed around 'k' is 'SKI'.
(Other definitions for ski that I've seen before include "Descend fast" , "Take part in a winter sport" , "go uphill? Probably not!" , "Winter sport item" , "Type of snow shoe" .)