A set of rooms; a flat (9)
I believe the answer is:
apartment
I believe this is a double definition.
'a set of rooms' is the first definition.
(I've seen this before)
'a flat' is the second definition.
(I know that flat can be written as apartment)
(Other definitions for apartment that I've seen before include "Room ? flat" , "property" , "Flat (mainly US)" , "Suite of rooms to live in" , "Penthouse" .)