Mother with large, fat duck (7)
I believe the answer is:
Here is my best explanation:
'duck' is the definition.
(mallard is a kind of duck)
'mother with large fat' is the subsidiary indication.
'mother' becomes 'ma' (ma is a kind of mother).
'with' means one lot of letters go next to another.
'large' becomes 'l'.
'fat' becomes 'lard' (lard is a kind of fat**).
(Other definitions for mallard that I've seen before include "Ancestor of all domestic ducks", "Type of duck - world speed record holding steam engine (1938)", "Oldest breed of duck", "Kind of duck", "Wild duck", "Engine holding the world speed record for a steam locomotive, set at 123mph in 1938", "Duck", "Variety of duck" and "Common wild duck".)