20 down
Put
9’s
head
(or
tail)
in
bank
vault
(7)
Put 9’s head (or tail) in bank vault (7)
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The definition is at the end of the clue.
How the clue works
The answer at 9 across is HEATH, and its head (or tail) is H — same letter either way. Pop that H inside CAMBER (a bank or slope) to get the vault.