Lengthy attack (5)
I believe the answer is:
siege
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lengthy attack</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)