The best thing on our menu? Flounders (6,3)
I believe the answer is:
numero uno
'the best thing' is the definition.
'on our menu? flounders' is the wordplay.
'flounders' is an anagram indicator (I've seen 'flounder' mean this).
'on'+'our'+'menu'='onourmenu'
'onourmenu' with letters rearranged gives 'NUMERO UNO'.
(Other definitions for numero uno that I've seen before include "Ace Italian" , "Big cheese from Italy" , "boss?" , "Berlusconi once" .)