Terminate a mission (5)
I believe the answer is:
abort
'terminate a mission' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abort that I've seen before include "Bring to a premature end" , "Call off (mission) because of problem" , "[FINISH] prematurely" , "Pull the plug on" , "Bring to premature conclusion" .)