New in constituency, old Republican politician (7)
I believe the answer is:
senator
'politician' is the definition.
(I've seen this before)
'new in constituency old republican' is the wordplay.
'new' becomes 'n'.
'in' means one lot of letters goes inside another.
'constituency' becomes 'seat' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'republican' becomes 'r' (abbreviation).
'n' inserted inside 'seat' is 'senat'.
'senat'+'o'+'r'='SENATOR'
(Other definitions for senator that I've seen before include "Governor" , "Upper house US Congressman" , "Legislator -- treason (anag)" , "Treason committed by member of Upper House" , "Statesman -- treason (anag)" .)