Prolonged battle (5)
I believe the answer is:
siege
'prolonged battle' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for siege that I've seen before include "Blockade of a town or fortress" , "Military operation cutting off access" , "Surrounding and blockading of town by military" , "Military operation surrounding a town" , "Offence in which fortified place is surrounded" .)