Rome turned over by invading monarch (7)
I believe the answer is:
emperor
'monarch' is the definition.
(I know that monarch can be written as emperor)
'rome turned over by invading' is the wordplay.
'turned over' says the letters should be written backwards.
'by' becomes 'per' (I've seen this in another clue).
'invading' indicates putting letters inside.
'rome' back-to-front is 'emor'.
'emor' enclosing 'per' is 'EMPEROR'.
(Other definitions for emperor that I've seen before include "Purple --; -- penguin" , "Powerful ruler" , "Japanese version of the king" , "Absolute ruler" , "Napoleon Bonaparte, for instance" .)