Murderous alter ego of fiction (4)
I believe the answer is:
hyde
'murderous alter ego of fiction' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hyde that I've seen before include "and 21 Across: Green area in London" , "See 15" , "First president" , "Manchester town, London park" , "doctor turned into during work" .)