Mum fat eating large duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(I know that mallard is a type of duck)
'mum fat eating large' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'fat' becomes 'lard' (lard is a kind of fat).
'eating' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'lard' going around 'l' is 'llard'.
'ma'+'llard'='MALLARD'
(Other definitions for mallard that I've seen before include "Ancestor of all domestic ducks" , "Kind of duck" , "web-footed bird" , "Type of duck - world speed record holding steam engine (1938)" , "Variety of duck" .)