One room and most of the outside had been demolished (6)
I believe the answer is:
Here is my best explanation:
'one room' is the definition.
'most of the outside had been demolished' is the subsidiary indication.
'most of' means to remove the last letter.
'had been demolished' indicates anagramming the letters.
'outside' with its last letter taken away is 'outsid'.
'outsid' with letters rearranged gives 'studio'.
'and' is the link.
(Other definitions for studio that I've seen before include "Where an artist works", "Elstree area", "Small apartment", "Building for movie-making", "Recording room", "Where one might find artists", "Artist's place of work", "Dance practice room", "Atelier, say?" and "Film producer".)