Significant distance for vehicle with slope underneath (9)
I believe the answer is:
wheelbase
'significant distance for vehicle' is the definition.
'with slope underneath' is the wordplay.
'with' becomes 'w' (abbreviation).
'slope' becomes 'heel'.
'underneath' becomes 'base'.
'w'+'heel'+'base'='WHEELBASE'
(Other definitions for wheelbase that I've seen before include "Length of most of car" , "Source of power" .)