24 down
Perhaps
Rosemary
is
writing
her
book
(4)
Perhaps Rosemary is writing her book (4)
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The definition is at the beginning of the clue.
How the clue works
See Perhaps Rosemary at the start — that's your definition, since rosemary is just one example of a herb. Then the answer's hiding in plain sight in "writing her book", tucked forwards inside HER-B.