Military mission (6)
I believe the answer is:
sortie
'military mission' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sortie that I've seen before include "Military aircraft's flight" , "Military aircraft flight" , "Raid - short trip" , "Breakout by besieged garrison" , "Operational flight by a bomber or fighter" .)