Declaration from senator is out of order (9)
I believe the answer is:
assertion
'declaration' is the definition.
(I know that assertion is a type of declaration)
'senator is out of order' is the wordplay.
'out of order' is an anagram indicator.
'senator'+'is'='senatoris'
'senatoris' anagrammed gives 'ASSERTION'.
'from' is the link.
(Other definitions for assertion that I've seen before include "Confident statement of fact" , "Claim" , "Notarises (anag)" , "Confident declaration" , "Emphatic declaration" .)