Summing up saying something further (6)
I believe the answer is:
adding
'summing' is the definition.
('add' can be a synonym of 'sum')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for adding that I've seen before include "Totalling" , "Contributing" , "Totting" , "increasing it by" , "Appending" .)