21 down
A
mortgage
said
to
be
unique
(5)
A mortgage said to be unique (5)
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The definition is at the end of the clue.
How the clue works
The definition is unique, giving you ALONE. You've got "A" straight off, then "mortgage" gives you LOAN, which when "said to be" (your homophone nudge) sounds like LONE — so A tacked onto LONE gets you there.