Vetch plant (4)
I believe the answer is:
tare
'vetch plant' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tare that I've seen before include "Biblical weed" , "Allowance made for the weight of the container in which goods are packed" , "Allowance for package weight; vetch" , "Weedy annual grass, often occurs in grainfields" , "Vehicle's empty weight" .)