Does sum work? (4)
I believe the answer is:
adds
'does sum work?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for adds that I've seen before include "Aggregates or appends" , "Puts in; tacks on" , "Calculates the sum" , "Goes on to say" , "Appends; Totals" .)