Excellent swimmer (5)

I believe the answer is:

brill

I believe this is a double definition.

'excellent' is the first definition.
(brill is short for brilliant)

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

(Other definitions for brill that I've seen before include "Very good - fish" , "Champion" , "Excellent (abbr) - fish" , "Fab -- fish" , "Flatfish; excellent" .)

I've seen this clue in The Telegraph and The Sun.

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