A Mrs Green ordering a duck (9)
I believe the answer is:
merganser
'a duck' is the definition.
(I know that merganser is a type of sea duck)
'a mrs green ordering' is the wordplay.
'ordering' is an anagram indicator.
'a'+'mrs'+'green'='amrsgreen'
'amrsgreen' with letters rearranged gives 'MERGANSER'.
(Other definitions for merganser that I've seen before include "Together saw bill" , "Diving bird" , "Diver" , "Diving duck" , "Sawbill" .)