Weed; the common vetch (4)
I believe the answer is:
tare
I believe this clue is a double definition.
'weed' is the first definition.
(I've seen this before)
'the common vetch' is the second definition.
(I've seen this in another clue)
(Other definitions for tare that I've seen before include "Variety of vetch" , "Allowance for package weight; vetch" , "Rate a vetch" , "Vessel's weight" , "Biblical weed" .)