Prolonged military attack (5)
I believe the answer is:
siege
'prolonged military attack' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for siege that I've seen before include "Operation to capture a fort, for example" , "Action that isolates and attacks" , "Blockade of a fortified place" , "Offence in which fortified place is surrounded" , "Military operation cutting off access" .)