By the sound of it, locate swimmer (6)
I believe the answer is:
plaice
'swimmer' is the definition.
(plaice is a kind of fish)
'by the sound of it locate' is the wordplay.
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'locate' becomes 'place' (placing is a kind of locating).
'place' is a homophone of 'PLAICE'.
(Other definitions for plaice that I've seen before include "Flatfish, valued as food" , "Flatfish; sounds like put" , "Large flat fish" , "Type of fish" , "N. Atlantic flatfish" .)