Queen formerly admitting former attachment (6)
I believe the answer is:
annexe
'attachment' is the definition.
(eg part attached to a building)
'queen formerly admitting former' is the wordplay.
'queen formerly' becomes 'Anne' (Queen Anne, 18th-century queen).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'former' becomes 'ex' (prefix meaning former or onetime).
'anne' enclosing 'ex' is 'ANNEXE'.
(Other definitions for annexe that I've seen before include "Adjoining building" , "Building attached to a larger one" , "Additional building" , "Extension, supplement" , "Building attached to a largerone" .)