Mathematical operation (8)
'mathematical operation' is the definition.
(addition is a kind of mathematical operation)
This is the entire clue.
(Other definitions for addition that I've seen before include "plus" , "Calculation of the sum of numbers" , "Calculation of the sum of two or more" , "Something extra" , "an arithmetical process" .)
(Other definitions for minus that I've seen before include "Negative sign - negative thing" , "'Take away' sign" , "Lacking; negative" , "Reduced by the subtraction of - thus?" , "Less, without" .)