Swimmer I spot outside (6)

I believe the answer is:

plaice

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'swimmer' is the definition.
(plaice is a kind of fish)

I can't explain the rest of the clue.

(Other definitions for plaice that I've seen before include "N. Atlantic flatfish" , "Ice, pal, will catch this fish" , "Large flatfish" , "Location of the fish" , "Type of fish" .)

I've seen this clue in The Sun.

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