Lair (3)
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'lair' is the definition.
(synonyms)
This is all the clue.
(Other definitions for den that I've seen before include "Animal's home or gent's study" , "Hidden lair of an animal" , "Wild animal's home" , "Animal's lair or man's study" , "Hiding place" .)
(Other definitions for pen that I've seen before include "Put down" , "See 20" , "Confine the writer" , "Animal enclosure" , "Writing tool" .)