Prevents spots coming back (5)
I believe the answer is:
stops
'prevents' is the definition.
(stopping is a kind of preventing)
'spots coming back' is the wordplay.
'coming back' shows that the letters should be reversed in order.
'spots' reversed gives 'STOPS'.
(Other definitions for stops that I've seen before include "Discontinues" , "Ceases" , "Ends" , "Prevents" , "Checks" .)