A little bird sitting comfortably (8)

I believe the answer is:

nestling

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

'a little bird' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for nestling that I've seen before include "getting under duvet?" , "Baby bird" , "Sitting snugly" , "Young bird not yet fledged" , "Little bird" .)

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