Poor snail one Yankee cooked with onions (9)
I believe the answer is:
lyonnaise
'cooked with onions' is the definition.
(I've seen this before)
'poor snail one yankee' is the wordplay.
'poor' indicates anagramming the letters.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'snail'+'one'+'y'='snailoney'
'snailoney' is an anagram of 'LYONNAISE'.
(Other definitions for lyonnaise that I've seen before include "Served with onions" , "cooking with onions" , "(Of potatoes) cooked with onions in a wine sauce" , "cooked in a way to make you cry?" .)