Log really concealed monster (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'log really concealed' is the wordplay.
'concealed' indicates the answer is hidden within the clue.
'OGRE' can be found hidden inside 'log really'.
(Other definitions for ogre that I've seen before include "Man-eating monster of fairy tales" , "One to gobble flesh" , "Cruel fairytale giant" , "Giant of folklore who ate humans" , "Cruel and wicked person" .)