Jekyll's evil alter ego (4)
I believe the answer is:
hyde
'jekyll's evil alter ego' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hyde that I've seen before include "Green space in London" , "... the latter side of [EVIL] nature" , "doctor turned into during work" , "The evil personality (see 12)" , "See 15" .)