Affirmation, declaration (9)
I believe the answer is:
This clue is a double definition.
'affirmation' is the first definition.
(I know that affirmation can be written as assertion)
'declaration' is the second definition.
(I know that assertion is a type of declaration)
(Other definitions for assertion that I've seen before include "Claim" , "Notarises (anag)" , "Emphatic declaration" , "Confident statement of fact" , "Confident declaration" .)