Plunder weapon (5)
I believe the answer is:
rifle
I believe this clue is a double definition.
'plunder' is the first definition.
('rifle' can be a synonym of 'plunder')
'weapon' is the second definition.
(rifle is a kind of weapon)
(Other definitions for rifle that I've seen before include "Ransack; a weapon" , "Ransack - firearm" , "Gun with a long barrel" , "Rummage" , "Security provider with stock" .)