Some insane Roman emperor? (4)
I believe the answer is:
nero
'emperor?' is the definition.
(Roman emperor)
'some insane roman' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'NERO' can be found hidden inside 'insane roman'.
(Other definitions for nero that I've seen before include "Emperor who played the fiddle" , "Famous fiddler" , "Emperor, 'burnt' Rome" , "Ancient Roman emperor" , "Roman 9" .)