Prolonged attack (5)
I believe the answer is:
siege
'prolonged 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 "seat" , "Persistent attempt to gain" , "Blockade of a town or fortress" , "Surrounding and blockading of town by military" , "Blockade of a fortified place" .)