Mother left Fat Duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(I know that mallard is a type of duck)
'mother left fat' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'left' becomes 'l' (common abbreviation).
'fat' becomes 'lard' (lard is a kind of fat).
'ma'+'l'+'lard'='MALLARD'
(Other definitions for mallard that I've seen before include "Ancestor of all domestic ducks" , "Variety of duck" , "Oldest breed of duck" , "Type of duck - world speed record holding steam engine (1938)" , "Kind of duck" .)