Sponsor accepts seat as politician (11)
I believe the answer is:
backbencher
'politician' is the definition.
(backbencher is a kind of politician)
'sponsor accepts seat' is the wordplay.
'sponsor' becomes 'backer' (someone backing or sponsoring something).
'accepts' indicates putting letters inside.
'seat' becomes 'bench' (bench is a kind of seat).
'backer' enclosing 'bench' is 'BACKBENCHER'.
'as' acts as a link.
(Other definitions for backbencher that I've seen before include "MP without office" , "MP not holding office" , "Rank and file MP" , "Representative" , "Legislator" .)