Senator is changing profession (9)
I believe the answer is:
assertion
'profession' is the definition.
(thesaurus)
'senator is changing' is the wordplay.
'changing' indicates anagramming the letters.
'senator'+'is'='senatoris'
'senatoris' anagrammed gives 'ASSERTION'.
(Other definitions for assertion that I've seen before include "Confident declaration" , "Confident statement of fact" , "Claim" , "Emphatic declaration" , "Notarises (anag)" .)