Ministry of Transport's alternative car (5)
I believe the answer is:
motor
'car' is the definition.
(motor can informally mean a car)
'ministry of transport's alternative' is the wordplay.
'ministry of transport' becomes 'mot' (this could be a standard abbreviation I've not previously seen).
'alternative' becomes 'or'.
'mot'+'or'='MOTOR'
Can you help me to learn more?
(Other definitions for motor that I've seen before include "Machine that converts energy into motion" , "Travel by car, engine" , "Driver" , "Internal combustion engine" , "Move fast" .)