Finds a sum (4)
I believe the answer is:
adds
'finds 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 "Sums, totals" , "Aggregates or appends" , "Appends or increases" , "Contributes further" , "Puts (numbers) together" .)