Professional farm vehicle used by geometry students (10)
I believe the answer is:
protractor
'professional farm vehicle used by geometry students' is the definition.
I can't tell whether this definition defines the answer.
'professional farm vehicle used by' is the wordplay.
'professional' becomes 'pro' (abbreviation).
'farm vehicle' becomes 'tractor' (vehicle used on a farm).
'used by' is a charade indicator (letters next to each other).
'pro'+'tractor'='PROTRACTOR'
Can you help me to learn more?
(Other definitions for protractor that I've seen before include "Geometrical instrument" , "Geometry set item" , "muscle" , "Angle-measuring device" , "draughtsman may use this" .)