Runners on this small step (6)
I believe the answer is:
sledge
'runners on this' is the definition.
(the parts of the sledge that touch the snow)
'small step' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'step' becomes 'ledge' (I have seen 'A step' mean 'ledge' so perhaps 'step' could also mean 'ledge').
's'+'ledge'='SLEDGE'
(Other definitions for sledge that I've seen before include "Vehicle mounted on runners" , "Transport on snow - hammer" , "Vehicle for transportation over snow" , "Put cricketer off" , "one could go downhill fast" .)