Scale button (4)
I believe the answer is:
tare
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">scale button</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)