Empty truck's weight (4)
I believe the answer is:
tare
'empty truck's weight' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tare that I've seen before include "Allowance made for the weight of the container in which goods are packed" , "Weight of container makes a vetch" , "Moved very quickly" , "Variety of vetch" , "Vehicle weight minus fuel and load" .)