Agent in European capital backed Julian, say (7)
I believe the answer is:
emperor
'julian say' is the definition.
(I know that Julian is a Roman emperor)
'agent in european capital backed' is the wordplay.
'agent' becomes 'rep' (short for representative).
'in' means one lot of letters goes inside another.
'european capital' becomes 'Rome' (capital of Italy).
'backed' says the letters should be written backwards.
'rep' put within 'rome' is 'rorepme'.
'rorepme' written backwards gives 'EMPEROR'.
(Other definitions for emperor that I've seen before include "Beethoven piano concerto, Op. 73" , "Ancient Roman ruler" , "eg Nero" , "Large colourful butterfly" , "Sovereign - ruler" .)