Mum left with fat duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(I know that mallard is a type of duck)
'mum left with fat' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'left' becomes 'l' (common abbreviation).
'with' means one lot of letters go next to another.
'fat' becomes 'lard' (lard is a kind of fat).
'ma'+'l'+'lard'='MALLARD'
(Other definitions for mallard that I've seen before include "Common wild duck" , "Variety of duck" , "Ancestor of all domestic ducks" , "Oldest breed of duck" , "Type of duck - world speed record holding steam engine (1938)" .)