Caesar's representative in Rome upset (7)
I believe the answer is:
emperor
'caesar's' is the definition.
(I've seen this in another clue)
'representative in rome upset' is the wordplay.
'representative' becomes 'rep' (rep is a kind of representative).
'in' means one lot of letters goes inside another.
'upset' shows that the letters should be reversed in order.
'rep' placed inside 'rome' is 'rorepme'.
'rorepme' backwards is 'EMPEROR'.
(Other definitions for emperor that I've seen before include "Powerful ruler" , "Roman leader" , "eg Nero" , "Character in 24 ac" , "Nero or penguin" .)