Scale button (4)
I believe the answer is:
tare
'scale button' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tare that I've seen before include "Rate a vetch" , "Common vetch" , "Moved very quickly" , "Weight of unloaded vehicle" , "Weedy annual grass, often occurs in grainfields" .)