Some retrogressive monster (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'some retrogressive' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'OGRE' can be found hidden inside 'retrogressive'.
(Other definitions for ogre that I've seen before include "The giant is some goer" , "Terrifying man" , "Has a monstrous role" , "Wicked giant" , "Monstrous person" .)