Lairs for bears (4)
'lairs for bears' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dens that I've seen before include "Habitats of some wild animals" , "Man's" , "Habitations of wild animals" , "Studies" , "Lairs" .)
(Another definition for caves that I've seen is " Underground chambers".)