Button on a deli scale (4)
I believe the answer is:
tare
'button on a deli scale' 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 "Common vetch" , "Vessel's weight" , "Moved very quickly" , "Biblical weed" , "Vehicle weight minus fuel and load" .)