Put iodine in swimmer (6)
I believe the answer is:
plaice
'swimmer' is the definition.
(plaice is a kind of fish)
'put iodine in' is the wordplay.
'put' becomes 'place' (synonyms).
'iodine' becomes 'I' (I is the chemical symbol for iodine).
'in' means one lot of letters goes inside another.
'place' placed around 'i' is 'PLAICE'.
(Other definitions for plaice that I've seen before include "Important food fish" , "Edible flatfish" , "Large flat fish" , "Tasty flatfish I place here" , "Position the fish, we hear" .)