Ruler has upset agent in capital (7)
I believe the answer is:
emperor
'ruler' is the definition.
(emperor is a kind of ruler)
'upset agent in capital' is the wordplay.
'upset' says the letters should be written backwards.
'agent' becomes 'rep' (short for representative).
'in' means one lot of letters goes inside another.
'capital' becomes 'rome' (capital of Italy).
'rep' placed inside 'rome' is 'rorepme'.
'rorepme' back-to-front is 'EMPEROR'.
'has' acts as a link.
(Other definitions for emperor that I've seen before include "chap such as Charlemagne" , "He rules over several states" , "Supreme ruler" , "Pre-Rome (anag.)" , "Napoleon Bonaparte, for instance" .)