Vehicle test for roofless car (5)
I believe the answer is:
motor
'car' is the definition.
(motor can informally mean a car)
'vehicle test for roofless' is the wordplay.
'vehicle test' becomes 'MOT' (MOT test).
'roofless' means to remove the first letter.
'for' with its initial letter removed is 'or'.
'mot'+'or'='MOTOR'
(Other definitions for motor that I've seen before include "Internal combustion engine" , "Driver" , "go quickly" , "(Transport using) engine" , "Car (colloq,)" .)