Container for plunder (4)
I believe the answer is:
I believe this clue is a double definition.
'container' is the first definition.
(I know that sack is a type of container)
'for plunder' is the second definition.
(Other definitions for sack that I've seen before include "Large hessian bag" , "Container for coal etc" , "destroy > 50s dress" , "Plundering of captured town" , "Dismissal - sherry!" .)