Jekyll's evil side (4)
I believe the answer is:
hyde
'jekyll's evil side' 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 "Green space in London" , "Manchester town, London park" , "The evil personality (see 12)" , "doctor turned into during work" , "and 21 Across: Green area in London" .)