How slim Woody is? (7)

I believe the answer is:

willowy

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'how slim' is the definition.
'willowy' can be an answer for 'slim' (thesaurus). I am not sure about the 'how' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for willowy that I've seen before include "Flexible" , "Like a slender person" .)

I've seen this clue in the Evening Standard.

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