One room and most of the outside had been demolished (6)
I believe the answer is:
studio
'one room' is the definition.
'most of the outside had been demolished' is the wordplay.
'most of' means to remove the last letter.
'had been demolished' indicates an anagram.
'outside' with its final letter taken off is 'outsid'.
'outsid' with letters rearranged gives 'STUDIO'.
'and' acts as a link.
(Other definitions for studio that I've seen before include "Workroom for 27 down" , "Type of apartment without a separate bedroom" , "Centre for movie-making" , "Place for the practice of an art" , "Workroom of an artist or photographer" .)