Roman emperor starts new era ruling obscenely (4)
I believe the answer is:
nero
'roman emperor' is the definition.
(I've seen this before)
'starts new era ruling obscenely' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'new era ruling obscenely' is 'NERO'.
(Other definitions for nero that I've seen before include "Barbarous Roman emperor" , "Matricidal emperor" , "He's reported to have fiddled while Rome burned" , "Roman Emperor notorious for vice and luxury" , "Famous fiddler" .)