Old invader that's employed in sacking (4)
I believe the answer is:
jute
I believe this is a double definition.
'old invader' is the first definition.
(the Jutes were a Germanic tribe)
'that's employed in sacking' is the second definition.
(sacking may be made from jute)
(Other definitions for jute that I've seen before include "Rough plant fibre" , "Fibre for ropes and mats - member of ancient Germanic tribe" , "Fibre-producing plant" , "Coarse fibre used in making sacks etc" , "Companion for Anglo-Saxon" .)