Jekyll's alter ego (4)
I believe the answer is:
hyde
'jekyll's alter ego' 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 "... 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" .)