Prolonged attacks (6)
I believe the answer is:
sieges
'prolonged attacks' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sieges that I've seen before include "Military actions of isolation and attack" , "military blockades" , "Persistent attacks on fortified places" , "Persistent attacks on fortied places" , "army investments" .)