A mathematical process (8)
'a mathematical process' is the definition.
(addition is a kind of mathematical process)
This is the entire clue.
(Other definitions for addition that I've seen before include "Calculation of the sum of numbers" , "Arithmetic calculation" , "Annexe" , "Mathematical process" , "One of the four basic planks of arithmetic" .)
(Other definitions for division that I've seen before include "Military grouping" , "Large army unit" , "some police?" , "Parliamentary vote" , "Section; maths process" .)