Lion's lair (3)
I believe the answer is:
'lion's lair' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for den that I've seen before include "Gentleman's study or animal's lair" , "Hidden lair of an animal" , "Animal's lair or gent's study" , "Animal's lair or man's study" , "Hideaway" .)