Hidden lair (3)
I believe the answer is:
den
'hidden lair' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for den that I've seen before include "Lair for animal or study for gent" , "Hiding place" , "Animal's lair or man's study" , "Animal's lair or gent's study" , "Wild animal's lair" .)