Swimmer with idiot on banks (6)
I believe the answer is:
wrasse
'swimmer' is the definition.
(wrasse is a kind of fish)
'with idiot on banks' is the wordplay.
'with' becomes 'w' (abbreviation).
'idiot' becomes 'ass' (I've seen this in other clues).
'on' becomes 're' ('on' can be mean 'with reference to').
'banks' means one lot of letters goes inside another (bank can mean to surround or border).
'ass' put into 're' is 'rasse'.
'w'+'rasse'='WRASSE'
(Other definitions for wrasse that I've seen before include "Swears about the tropical marine fish with strong teeth" , "Fish - swears (anag)" , "Bright marine fish" , "Swimmer" , "Brightly-coloured fish" .)