Calculates a total (4)
I believe the answer is:
adds
'calculates a total' 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 "Puts (numbers) together" , "Reckons" , "Appends or increases" , "Finds the sum (4)," , "Puts in; tacks on" .)