It measures the angles in favour of a powerful motor vehicle (10)
I believe the answer is:
protractor
'it measures the angles in favour' is the definition.
The definition suggests a singular noun which matches the answer.
'a powerful motor vehicle' is the wordplay.
'a powerful' becomes 'pro' (I can't explain this - if you can you should give a lot more credence to this answer).
'motor vehicle' becomes 'tractor' (tractor is a kind of motor vehicle).
'pro'+'tractor'='PROTRACTOR'
'of' is the link.
Can you help me to learn more?
(Other definitions for protractor that I've seen before include "Geometry set item" , "Geometrical instrument" , "muscle" , "draughtsman may use this" , "Angle-measuring device" .)