One Roman's emperor (4)
I believe the answer is:
nero
This clue is a double definition.
'one roman's' is the first definition.
(Nero was a Roman emperor)
'emperor' is the second definition.
(Roman emperor)
(Other definitions for nero that I've seen before include "Emperor said to have started the fire that destroyed Rome" , "Roman Emperor, 54-68 AD" , "Notorious Roman emperor, d. AD 68" , "Emperor (who fiddled while Rome burned?) d. AD68" , "Roman emperor who fiddled while Rome burned" .)