Sound situation for a swimmer at sea (6)
I believe the answer is:
plaice
'a swimmer at sea' is the definition.
(fish)
'sound situation' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'situation' becomes 'place' (place is a kind of situation).
'place' sounds like 'PLAICE'.
'for' acts as a link.
(Other definitions for plaice that I've seen before include "Ice, pal, will catch this fish" , "Large flat fish" , "Brown flatfish" , "Flatfish, valued as food" , "A succulent flat-fish" .)