Mother left with fat bird (7)
I believe the answer is:
mallard
'bird' is the definition.
(mallard is a kind of bird)
'mother left with fat' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'left' becomes 'l' (common abbreviation).
'with' says to put letters next to each other.
'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" , "Type of duck - world speed record holding steam engine (1938)" , "The famous steam train" , "Kind of duck" , "Ancestor of all domestic ducks" .)