Lengthy attack (5)
I believe the answer is:
siege
'lengthy 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 "Prolonged attack on a fortified place" , "Military operation to seal off (town)" , "Operation to capture a fort, for example" , "Starving-out operation" , "Persistent attempt to gain" .)