Sondheim's "Sweeney __" (4)
I believe the answer is:
todd
'sondheim's sweeney' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for todd that I've seen before include "Demon 12 of Fleet Street" , "19th-century demon barber of Fleet Street" , "Sweeney, Fleet Street barber" , "Irish-born British actor and soldier, Richard, d. 2009" , "Richard --, was Guy Gibson in The Dam Busters" .)