Calculates a sum (4)
I believe the answer is:
adds
'calculates a sum' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adds that I've seen before include "Tots up" , "Puts in; tacks on" , "Finds the sum (4)," , "Joins to something else" , "Supplements" .)