Long battle (5)
I believe the answer is:
siege
'long 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 "Operation to surround and cut off supplies" , "Military operation to seal off (town)" , "Operation to surround a town/fortress" , "Act of surrounding a town or fortress" , "Investment (of town)" .)