Ruler has upset agent in capital (7)
I believe the answer is:
emperor
'ruler' is the definition.
(I know that emperor is a type of crowned head)
'upset agent in capital' is the wordplay.
'upset' is a reversal indicator.
'agent' becomes 'rep' (short for representative).
'in' means one lot of letters goes inside another.
'capital' becomes 'rome' (capital of Italy).
'rep' going within 'rome' is 'rorepme'.
'rorepme' back-to-front is 'EMPEROR'.
'has' is the 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" .)