The only one caught? (4)

I believe the answer is:

sole

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'the only one' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for sole that I've seen before include "Unique" , "fish > that's never less than foot in length" , "One and only; part of foot" , "Dover - -" , "fish in the sea!" .)

I've seen this clue in the Evening Standard.

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