Swimmer takes position restricting current (6)
I believe the answer is:
plaice
'swimmer' is the definition.
(plaice is a kind of fish)
'position restricting current' is the wordplay.
'position' becomes 'place' (synonyms).
'restricting' is an insertion indicator.
'current' becomes 'I' (symbol for electric current).
'place' placed around 'i' is 'PLAICE'.
'takes' acts as a link.
(Other definitions for plaice that I've seen before include "Large flatfish" , "Tasty flatfish I place here" , "Flatfish, valued as food" , "Important food fish" , "Brown flatfish" .)