Prevent from happening (6)
(Other definitions for kibosh that I've seen before include "Finish off" , "Put this on something and stop it from happening" , "Fatal to put this on" , "Terminate" , "Make impossible to happen" .)
'prevent from happening' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for avert that I've seen before include "Head off" , "Fend off" , "Prevent from occurring" , "Deflect away" , "Turn aside, perhaps gaze" .)
(Other definitions for forestall that I've seen before include "Arrive first" , "Take action before" , "Anticipate" , "Obstruct by taking advance action" , "Check" .)