Heads off, prevents from happening (6)
I believe the answer is:
averts
I believe this clue is a double definition.
'heads off' is the first definition.
('avert' can be a synonym of 'head off')
'prevents from happening' is the second definition.
(I've seen this before)
(Other definitions for averts that I've seen before include "Turns away or causes to turn away" , "Heads off, prevents from happening" , "Deflects, heads off" , "Stops" , "Wards off" .)