In a whirl, sounds a bit unsteady (4)

I believe the answer is:

eddy

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'in' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

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

Can you help me to learn more?

(Other definitions for eddy that I've seen before include "current running back" , "Whirling current" , "Fellow" , "Little whirlpool" , "Swirl; Duane -, US guitarist" .)

I've seen this clue in the Evening Standard.

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