She's classically slim (5)

I believe the answer is:

sylph

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'she's' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sylph that I've seen before include "Slender, graceful female" , "Graceful lass" , "Slim woman" , "Thin and graceful female" , "Thin, graceful woman" .)

I've seen this clue in the Evening Standard.

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