Roman official (7)
'roman official' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for senator that I've seen before include "Member of the Upper House of the US Congress" , "Treason committed by member of Upper House" , "Member of a governing body" , "Clinton, perhaps" , "Member of government" .)
(Other definitions for tribune that I've seen before include "Champion of popular rights" , "Champion of the people" , "Senior officer in a Roman legion" , "Ancient Roman official, church dais" , "Popular demagogue" .)