Sound position for a swimmer (6)
I believe the answer is:
plaice
'for a swimmer' is the definition.
(plaice is a kind of fish)
'sound position' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'position' becomes 'place' (place is a kind of position).
'place' sounds like 'PLAICE'.
(Other definitions for plaice that I've seen before include "Flat fare" , "Flatfish; sounds like put" , "A succulent flat-fish" , "Type of fish" , "Location of the fish" .)