Roman magistrate (7)
'roman magistrate' is the definition.
This is the entire clue.
(Other definitions for tribune that I've seen before include "Official upholder of the people's rights" , "Institution upholding public rights" , "Roman people's official" , "Senior officer in a Roman legion" , "basilica" .)
(Other definitions for prefect that I've seen before include "Pupil with authority" , "Schoolchild official" , "Chief admin officer of a district in some countries" , "Chief officer (in France) - senior pupil" , "French official and senior pupil" .)