Legislator's appalling treason (7)
I believe the answer is:
senator
'legislator's' is the definition.
(senator is a kind of legislator)
'appalling treason' is the wordplay.
'appalling' indicates anagramming the letters (I've seen this in other clues).
'treason' with letters rearranged gives 'SENATOR'.
(Other definitions for senator that I've seen before include "Upper house US Congressman" , "American politician" , "One from uni's governing body" , "Member of upper house of legislature" , "Treason committed by member of Upper House" .)