An airy spirit, one imagines (5)

I believe the answer is:

sylph

I'm a little stuck... Click here to teach me more about this clue!

'spirit one imagines' is the definition.
'sylph' can be an answer for 'spirit' (I've seen this before). I'm not certain of the 'one imagines' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sylph that I've seen before include "Slender and graceful young woman" , "Thin, graceful woman" , "Imaginary spirit of the air" , "Girl with slender figure" , "Slender, graceful girl" .)

I've seen this clue in the Evening Standard.

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