Old queen gains unknown attachment (5)
I believe the answer is:
annex
'attachment' is the definition.
(I know that attachment can be written as annex)
'old queen gains unknown' is the wordplay.
'old queen' becomes 'Anne' (Queen Anne, 18th-century queen).
'gains' means one lot of letters go next to another.
'unknown' becomes 'x' (algebra).
'anne'+'x'='ANNEX'
(Other definitions for annex that I've seen before include "Grab" , "Tack on" , "Attach (land)" , "Attach, invade" , "An addition that extends a main building" .)