Crossword Genius

Sole - yet not the only one (8)

Ross

I believe the answer is:

flatfish

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'sole' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for flatfish that I've seen before include "Plaice or sole, for example" , "Sole, plaice etc" , "Swimmer" , "Flounder, perhaps" , "Perhaps sole" .)

I've seen this clue in The Globe and Mail.
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