Does some sums (4)
I believe the answer is:
adds
'does some sums' 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 "Goes on to say" , "Appends or increases" , "Joins to something else" , "Puts (numbers) together" , "Appends; Totals" .)