Girl transplanting a thin tree (9)

I believe the answer is:

henrietta

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'girl transplanting' is the definition.
The definition suggests a singular noun which matches the answer.

'a thin tree' is the wordplay.
'a thin tree' anagrams to 'HENRIETTA'.
But, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for henrietta that I've seen before include "girl" , "- Maria, wife of Charles I" .)

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