Queen describing former extension (6)
I believe the answer is:
annexe
'extension' is the definition.
('annexe' can be a synonym of 'extension')
'queen describing former' is the wordplay.
'queen' becomes 'anne' (I've seen this before).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'former' becomes 'ex'.
'anne' going around 'ex' is 'ANNEXE'.
(Other definitions for annexe that I've seen before include "Extra accommodation block" , "Side building" , "Extension, supplement" , "Addition that extends a main building" , "Joined-on building" .)