Agent entering capital, backing monarch (7)
I believe the answer is:
emperor
'monarch' is the definition.
(I know that emperor is a type of monarch)
'agent entering capital backing' is the wordplay.
'agent' becomes 'rep' (short for representative).
'entering' indicates putting letters inside.
'capital' becomes 'rome' (capital of Italy).
'backing' says the letters should be written in reverse.
'rep' going within 'rome' is 'rorepme'.
'rorepme' in reverse letter order is 'EMPEROR'.
(Other definitions for emperor that I've seen before include "chap such as Charlemagne" , "Supreme ruler" , "Character in 24 ac" , "Japanese version of the king" , "Sovereign and richly-colored butterfly" .)