Old queen holds appendix (6)
I believe the answer is:
annexe
'appendix' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'old queen holds' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'queen' becomes 'anne' (Queen Anne, 18th-century queen).
'holds' is an insertion indicator.
'ex' put within 'anne' is 'ANNEXE'.
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Added section of document" , "Additional room etc for house" , "An addition to a document or building" , "outbuilding" , "Attached (set of) room(s)" .)