Sovereign in capital keeping agent up (7)
I believe the answer is:
emperor
'sovereign' is the definition.
(emperor is a type of sovereign ruler)
'capital keeping agent up' is the wordplay.
'capital' becomes 'rome' (capital of Italy).
'keeping' is an insertion indicator.
'agent' becomes 'rep' (short for representative).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'rome' going around 'rep' is 'rorepme'.
'rorepme' backwards is 'EMPEROR'.
'in' is the link.
(Other definitions for emperor that I've seen before include "Powerful ruler" , "Beethoven's Piano Concerto No. 5 in E-flat major" , "Large richly-coloured butterfly" , "Sovereign - ruler" , "eg Nero" .)