Mum left with fat bird (7)
I believe the answer is:
mallard
'bird' is the definition.
(mallard is a kind of bird)
'mum left with fat' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'left' becomes 'L' (common abbreviation).
'with' is a charade indicator (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 "Kind of duck" , "web-footed bird" , "Ancestor of all domestic ducks" , "Type of duck - world speed record holding steam engine (1938)" , "Oldest breed of duck" .)