Originally nefarious emperor, Roman one -- that's him! (4)
I believe the answer is:
nero
'him' is the definition.
'originally nefarious emperor roman one' is the wordplay.
'originally' says to take the initial letters.
The initial letters of 'nefarious emperor roman one' is 'NERO'.
'that's' is the link.
(Other definitions for nero that I've seen before include "Wicked fiddler" , "Barbarous Roman emperor" , "Emperor said to have started the fire that destroyed Rome" , "Roman emperor on the fiddle?" , "He's reputed to have fiddled while Rome burned" .)