Villain's den (4)
I believe the answer is:
lair
'villain's den' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lair that I've seen before include "Wild animals' habitation" , "Place to hide" , "Private place of seclusion" , "Hiding place, den" , "Hideaway, den" .)