Discussed obscure monster of literature (4)
I believe the answer is:
hyde
'monster of literature' is the definition.
(Robert Louis Stevenson novel)
'discussed obscure' is the wordplay.
'discussed' shows a homophone (sound like).
'obscure' becomes 'hide' (synonyms).
'hide' sounds like 'HYDE'.
(Other definitions for hyde that I've seen before include "... the latter side of [EVIL] nature" , "Green space in London" , "Jekyll's alter ego" , "Manchester town, London park" , "First president" .)