Emperor in Wagner opera (4)
I believe the answer is:
nero
'emperor' is the definition.
(Roman emperor)
'in wagner opera' is the wordplay.
'in' indicates a hidden word.
'NERO' is hidden in the letters of 'wagner opera'.
(Other definitions for nero that I've seen before include "He's reported to have fiddled while Rome burned" , "Emperor said to have fiddled as Rome burned" , "Cruel Roman emperor" , "Roman 9" , "Emperor said to have started the fire that destroyed Rome" .)