Light scooter, if tested, keeps reversing (3,4,2)
I believe the answer is:
set fire to
'light' is the definition.
(thesaurus)
'scooter if tested keeps reversing' is the wordplay.
'keeps' indicates a hidden word.
'reversing' says the letters should be written in reverse.
'oteriftes' is hidden in the letters of 'scooter if tested'.
'oteriftes' written backwards gives 'SET FIRE TO'.
(Other definitions for set fire to that I've seen before include "torch" , "Ignite" , "Light" .)