Old woman left with fat duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(I know that mallard is a type of duck)
'old woman left with fat' is the wordplay.
'old woman' becomes 'ma' (both can mean someone's 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 "Kind of duck" , "Common wild duck" , "Ancestor of all domestic ducks" , "Oldest breed of duck" , "Type of duck - world speed record holding steam engine (1938)" .)