Measurer for vehicle (10)
I believe the answer is:
protractor
'measurer' is the definition.
(a protractor is used to measure angles)
'for vehicle' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'vehicle' becomes 'tractor' (tractor is a kind of vehicle).
'pro'+'tractor'='PROTRACTOR'
(Other definitions for protractor that I've seen before include "Geometrical instrument" , "Angle-measuring device" , "Geometry set item" , "muscle" , "draughtsman may use this" .)