Politician is not arse, surprisingly (7)
I believe the answer is:
senator
'politician' is the definition.
(I know that senator is a type of legislator)
'not arse surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'not'+'arse'='notarse'
'notarse' anagrammed gives 'SENATOR'.
'is' acts as a link.
(Other definitions for senator that I've seen before include "Legislative body member" , "politician going wrong" , "Upper house US Congressman" , "Member of Upper House engaged in treason" , "Member of upper house of legislature" .)