Swimmer one put outside (6)
I believe the answer is:
plaice
'swimmer' is the definition.
(plaice is a kind of fish)
'one put outside' is the wordplay.
'one' becomes 'i' (roman numeral).
'put' becomes 'place' (synonyms).
'outside' is an insertion indicator.
'i' inserted inside 'place' is 'PLAICE'.
(Other definitions for plaice that I've seen before include "Brown flatfish" , "Flatfish, valued as food" , "The fish sounds as if it hits the spot" , "Swimmer" , "Tasty flatfish I place here" .)