Talk of fish and duck (5)
I believe the answer is:
lingo
'talk' is the definition.
(thesaurus)
'fish and duck' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish**).
'and' means one lot of letters go next to another.
'duck' becomes 'o'.
'ling'+'o'='LINGO'
'of' is the link.
(Other definitions for lingo that I've seen before include "speech" , "Argot" , "special words" , "French?" , "strange language" .)