Sums up (4)
I believe the answer is:
adds
'sums up' is the definition.
('add' can be a synonym of 'sum up')
This is the entire clue.
(Other definitions for adds that I've seen before include "Contributes further" , "Finds the sum (4)," , "Reckons" , "Sums, totals" , "Does summer job" .)