Roadworthiness test for defaced car (5)
I believe the answer is:
motor
'car' is the definition.
(motor can informally mean a car)
'roadworthiness test for defaced' is the wordplay.
'roadworthiness test' becomes 'mot' (I can't justify this - if you can you should give a lot more credence to this answer).
'defaced' means to remove the first letter (the word with its face removed).
'for' with its first letter taken off is 'or'.
'mot'+'or'='MOTOR'
Can you help me to learn more?
(Other definitions for motor that I've seen before include "Car (informal)" , "Move quickly" , "Driver" , "Engine - relating to movement" , "Machine that converts energy into motion" .)