Fantastic swimmer (5)
I believe the answer is:
brill
'swimmer' is the definition.
(brill is a kind of fish)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for brill that I've seen before include "Excellent (abbr) - fish" , "First-class (informal) - fish" , "swimmer" , "marvellous" , "Very good - fish" .)