Emperor after Claudius (4)
I believe the answer is:
nero
'emperor after claudius' 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 "Roman emperor who fiddled while Rome burned" , "Emperor (who fiddled while Rome burned?) d. AD68" , "Ruler" , "Cruel Roman emperor" , "Imperial Roman fiddler" .)