Jargon for 'swimmer with nothing on' (5)
I believe the answer is:
lingo
'jargon' is the definition.
(I know that jargon can be written as lingo)
'swimmer with nothing on' is the wordplay.
'swimmer' becomes 'ling' (ling is a type of fish).
'with' is a charade indicator (letters next to each other).
'nothing on' becomes 'o' (I've seen this in other clues).
'ling'+'o'='LINGO'
'for' acts as a link.
(Other definitions for lingo that I've seen before include "Log in (anag.)" , "Jargon, language (colloq.)" , "speech" , "strange language" , "Tongue" .)