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 wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'with' is a charade indicator (letters next to each other).
'large' becomes 'l'.
'fat' becomes 'lard' (I've seen this before).
(Other definitions for mallard that I've seen before include "Duck", "web-footed bird", "Variety of duck", "Kind of duck", "Ancestor of all domestic ducks", "Train", "Common wild duck", "variety", "Ducks" and "Type of duck - world speed record holding steam engine (1938)".)