Small shelf for vehicle mounted on runners (6)
I believe the answer is:
sledge
'vehicle mounted on runners' is the definition.
(I've seen this before)
'small shelf' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'shelf' becomes 'ledge' (synonyms).
's'+'ledge'='SLEDGE'
'for' acts as a link.
(Other definitions for sledge that I've seen before include "one could go downhill fast" , "Transport on snow - hammer" , "heavy tool" , "Vehicle for use in snow" , "Narrow cart with runners for travelling on snow" .)