Senator is making a claim (9)
I believe the answer is:
assertion
'a claim' is the definition.
(I've seen this before)
'senator is making' is the wordplay.
'making' indicates an anagram.
'senator'+'is'='senatoris'
'senatoris' anagrammed gives 'ASSERTION'.
(Other definitions for assertion that I've seen before include "Claim" , "Emphatic declaration" , "Notarises (anag)" , "Confident declaration" , "Confident statement of fact" .)