Monster involved in rather gory comeback (4)
I believe the answer is:
ogre
'monster' is the definition.
(synonyms)
'involved in rather gory comeback' is the wordplay.
'involved in' says the answer is hidden in the clue.
'comeback' says the letters should be written in reverse.
'ergo' is hidden in the letters of 'rather gory'.
'ergo' backwards is 'OGRE'.
(Other definitions for ogre that I've seen before include "One to gobble flesh" , "Man-eating monster of fairy tales" , "Fairy-tale creature" , "Cruel giant of folklore" , "Giant: monster" .)