Roman Emperor notorious for vice and luxury (4)
I believe the answer is:
nero
'roman emperor notorious for vice and luxury' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nero that I've seen before include "Roman arsonist and fiddler" , "Roman emperor on the fiddle?" , "Ruler" , "Roman emperor who fiddled while Rome burned" , "He's reputed to have fiddled while Rome burned" .)