Monster conger eel oddly vanishing (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'conger eel oddly vanishing' is the wordplay.
'oddly vanishing' indicates alternate letters (letters in odd-numbered positions removed).
'conger'+'eel'='congereel'
The alternate letters of 'congereel' are 'OGRE'.
(Other definitions for ogre that I've seen before include "Monstrous person" , "Giant of folklore who liked to eat humans" , "Giant: monster" , "One to gobble flesh" , "Cruel and wicked person" .)