Attack tactic (5)
I believe the answer is:
siege
'attack tactic' 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 "Persistent attempt to gain" , "Blockade of a fortified place" , "Offence in which fortified place is surrounded" , "Prolonged attack on a fortified place" , "Operation to surround and cut off supplies" .)