Head off
'head off' is the definition.
(both can mean to prevent)
This is all the clue.
(Other definitions for avert that I've seen before include "Parry" , "Turn aside, prevent" , "Turn away or aside" , "Turn away, prevent" , "Forestall" .)
(Other definitions for depart that I've seen before include "Go away or leave" , "Go away; deviate" , "Retire from" , "Leave, vanish" , "Start out" .)