Sudden attack by besieged troops (6)
I believe the answer is:
sortie
'sudden attack by besieged troops' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sortie that I've seen before include "Military aircraft flight" , "Short trip; attack" , "Sally" , "Warplane's flight" , "Breakout by besieged garrison" .)