Queen defends old extension (6)
I believe the answer is:
annexe
'extension' is the definition.
('annexe' can be a synonym of 'extension')
'queen defends old' is the wordplay.
'queen' becomes 'anne' (I've seen this before).
'defends' means one lot of letters goes inside another.
'old' becomes 'ex'.
'anne' placed around 'ex' is 'ANNEXE'.
(Other definitions for annexe that I've seen before include "Small building added to another" , "Additional building" , "Supplementary part" , "Attached (set of) room(s)" , "outbuilding" .)