Scrap a vehicle test requiring reversing (4)
I believe the answer is:
atom
'scrap' is the definition.
(I've seen this before)
'a vehicle test requiring reversing' is the wordplay.
'vehicle test' becomes 'MOT' (MOT test).
'requiring reversing' shows that the letters should be reversed in order.
'mot' written backwards gives 'tom'.
'a'+'tom'='ATOM'
(Other definitions for atom that I've seen before include "Smallest component of an element" , "Tiny bit" , "A mighty particle, makes huge explosion" , "Smallest unit of matter" , "Particle; molecule" .)