Insanely difficult to eat duck cooked with onions (9)
I believe the answer is:
lyonnaise
'cooked with onions' is the definition.
(I've seen this before)
'insanely difficult to eat duck' is the wordplay.
'difficult' indicates anagramming the letters.
'to eat' indicates putting letters inside.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'insanely' is an anagram of 'lynnaise'.
'lynnaise' enclosing 'o' is 'LYONNAISE'.
(Other definitions for lyonnaise that I've seen before include "Served with onions" , "(Of potatoes) cooked with onions in a wine sauce" , "cooking with onions" , "cooked in a way to make you cry?" .)