Stop terrorist head backing state (5)
I believe the answer is:
avert
'stop' is the definition.
(thesaurus)
'terrorist head backing state' is the wordplay.
'head' says to take the initial letters.
'backing' says to put letters next to each other.
'state' becomes 'aver' (I've seen this before**).
The first letter of 'terrorist' is 't'.
't' put after 'aver' is 'AVERT'.
(Other definitions for avert that I've seen before include "Turn aside, perhaps gaze" , "Prevent from happening" , "Prevent, ward off" , "Turn away from, say gaze" , "Turn away, prevent" .)