Addition (10)
'addition' is the definition.
(supplement is a kind of addition)
This is the entire clue.
(Other definitions for supplement that I've seen before include "Extra" , "addition" , "additive" , "Add-on" , "Boost" .)
'addition' is the definition.
(I know that addition can be written as summation)
This is all the clue.
(Other definitions for summation that I've seen before include "Synopsis" , "Adding process" .)