Jekyll's alter ego (4)
I believe the answer is:
hyde
'jekyll's alter ego' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hyde that I've seen before include "... the latter side of [EVIL] nature" , "Green space in London" , "Manchester town, London park" , "and 21 Across: Green area in London" , "Jekyll's alter ego" .)