Kelly's back room (3)
I believe the answer is:
den
'room' is the definition.
(den is a kind of room)
'kelly's back' is the wordplay.
'kelly' becomes 'ned' (outlaw Ned Kelly).
'back' says the letters should be written backwards.
'ned' reversed gives 'DEN'.
(Other definitions for den that I've seen before include "Private office" , "Lair for animal or study for gent" , "Miserable place" , "Animal habitation" , "Habitation of wild animals (3)" .)