Seen in fog, really hideous monster (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'seen in fog really hideous' is the wordplay.
'seen in' indicates the answer is hidden within the clue.
'OGRE' can be found hidden inside 'fog really hideous'.
(Other definitions for ogre that I've seen before include "Monstrous creature" , "Terrifying man" , "Cruel fairytale giant" , "Terrifying person" , "Fierce giant" .)