Constituency or housing association's ultimate politician (7)
I believe the answer is:
senator
'politician' is the definition.
(I know that senator is a type of legislator)
'constituency or housing association's ultimate' is the wordplay.
'constituency' becomes 'seat' (House of Commons seats represent constituencies).
'housing' indicates putting letters inside.
'ultimate' says to take the final letters.
The final letter of 'association' is 'n'.
'seat'+'or'='seator'
'seator' enclosing 'n' is 'SENATOR'.
(Other definitions for senator that I've seen before include "Upper House member involved in treason" , "Governor" , "Legislative body member" , "Member of the Second House" , "One of 100 on Capitol Hill" .)