The fog really hides the beast! (4)
I believe the answer is:
ogre
'beast' is the definition.
(I've seen this before)
'the fog really hides' is the wordplay.
'hides' indicates the answer is hidden within the clue.
'OGRE' is hidden in the letters of 'fog really'.
'the' acts as a link.
(Other definitions for ogre that I've seen before include "Grim(m) beast?" , "Terrifying monster" , "The giant is some goer" , "Monster, troll" , "Has a monstrous role" .)