Supporter saving seat for politician (11)
I believe the answer is:
backbencher
'politician' is the definition.
(backbencher is a kind of politician)
'supporter saving seat' is the wordplay.
'supporter' becomes 'backer' (backer is a kind of supporter).
'saving' indicates putting letters inside.
'seat' becomes 'bench' (bench is a kind of seat).
'backer' placed around 'bench' is 'BACKBENCHER'.
'for' acts as a link.
(Other definitions for backbencher that I've seen before include "MP without office" , "Representative" , "Legislator" , "Rank and file MP" , "MP not holding office" .)