A growing girl (3)

I believe the answer is:

rue

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'a growing girl' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

This is all the clue.

Can you help me to learn more?

(Other definitions for rue that I've seen before include "The 'herb of grace'" , "Regret - wild shrub" , "Bitterly regret - plant" , "Feel regret for the herb" , "Bemoan" .)

I've seen this clue in the Evening Standard.

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