Sovereign's agent in capital sent back (7)
I believe the answer is:
emperor
'sovereign's' is the definition.
(emperor is a kind of sovereign)
'agent in capital sent back' is the wordplay.
'agent' becomes 'rep' (short for representative).
'in' indicates putting letters inside.
'capital' becomes 'rome' (Rome is an example).
'sent back' shows that the letters should be reversed in order.
'rep' put into 'rome' is 'rorepme'.
'rorepme' reversed gives 'EMPEROR'.
(Other definitions for emperor that I've seen before include "Beethoven piano concerto, Op. 73" , "Powerful ruler" , "Large richly-coloured butterfly" , "Absolute ruler" , "Kaiser" .)