Back part of flat is debutante's accommodation (6)
I believe the answer is:
bedsit
'accommodation' is the definition.
(I've seen this before)
'back part of flat is debutante's' is the wordplay.
'back' shows that the letters should be reversed in order.
'part of' says the answer is hidden in the clue.
'tisdeb' is hidden in the letters of 'flat is debutantes'.
'tisdeb' written backwards gives 'BEDSIT'.
(Other definitions for bedsit that I've seen before include "Small residence" , "Small flat" , "Simple accommodation" , "Rented room with all facilities" , "Cheap digs" .)