Fish served with duck's tongue (5)
I believe the answer is:
lingo
'tongue' is the definition.
(I've seen this before)
'fish served with duck's' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'served with' means one lot of letters go next to another.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'ling'+'o'='LINGO'
(Other definitions for lingo that I've seen before include "French?" , "Tongue" , "Unfamiliar language or jargon" , "Parlance" , "Language (informally)" .)