Totalling (7)
'totalling' is the definition.
('sum' can be a synonym of 'total')
This is all the clue.
(Another definition for summing that I've seen is " Calculating".)
'totalling' is the definition.
('add' can be a synonym of 'total')
This is the entire clue.
(Other definitions for adding that I've seen before include "Totting" , "Throwing in" , "Totalling" , "Putting in" , "Appending" .)