Mother with large, fat duck (7)
I believe the answer is:
mallard
'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).
'ma'+'l'+'lard'='MALLARD'
(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" , "The famous steam train" .)