They are pretty in a kettle - pretty muddled! (4)

I believe the answer is:

fish

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'they are' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for fish that I've seen before include "Burbot or turbot, for example" , "Aquatic animal" , "Ruff" , "Swimmer" , "Aquatic creature; rail-joining plate" .)

I've seen this clue in The Irish News.

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