Fifth Roman emperor (4)
I believe the answer is:
nero
'fifth roman emperor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nero that I've seen before include "Famous fiddler" , "Fiddling Roman emperor" , "Roman emperor, d. AD68" , "Roman emperor who fiddled while Rome burned" , "Barbarous Roman emperor" .)