Scrap a safety test, on reflection (4)
I believe the answer is:
atom
'scrap' is the definition.
(both can mean a tiny amount)
'a safety test on reflection' is the wordplay.
'safety test' becomes 'MOT' (MOT test for a vehicle).
'on reflection' says the letters should be written backwards.
'mot' backwards is 'tom'.
'a'+'tom'='ATOM'
(Other definitions for atom that I've seen before include "Neutron and proton" , "Very small object or amount" , "A tiny particle of matter with a big bang" , "A tiny powerful particle" , "Piece of matter in moat" .)