Queen outside old building (6)
I believe the answer is:
annexe
'building' is the definition.
(I've seen this before)
'queen outside old' is the wordplay.
'queen' becomes 'anne' (I've seen this before).
'outside' indicates putting letters inside.
'old' becomes 'ex'.
'anne' enclosing 'ex' is 'ANNEXE'.
(Other definitions for annexe that I've seen before include "Added section of document" , "Addition that extends a main building" , "Additional building" , "Appropriate into one's territory" , "Wing - extension" .)