Jekyll's counterpart (4)
I believe the answer is:
hyde
'jekyll's counterpart' 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 evil personality (see 12)" , "See 15" , "doctor turned into during work" , "... the latter side of [EVIL] nature" , "Jekyll's alter ego" .)