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