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' shows that the letters should be reversed in order.
'ned' back-to-front is '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)" .)