Queen by eastern extension (6)
I believe the answer is:
annexe
'extension' is the definition.
('annexe' can be a synonym of 'extension')
'queen by eastern' is the wordplay.
'queen' becomes 'anne' (I've seen this before).
'by' becomes 'x' (multiplication sign).
'eastern' becomes 'e'.
'anne'+'x'+'e'='ANNEXE'
(Other definitions for annexe that I've seen before include "Supplementary attached building" , "Additional room etc for house" , "Building joined to a larger one" , "Wing - extension" , "Attached (set of) room(s)" .)