Warplane mission (6)
I believe the answer is:
sortie
'warplane 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 manoeuvre" , "Breakout by besieged garrison" , "Raid - short trip" , "Warplane's flight" , "Military aircraft flight" .)