Mother left with fat duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(I know that mallard is a type of duck)
'mother left with fat' is the wordplay.
'mother' 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 "Engine holding the world speed record for a steam locomotive, set at 123mph in 1938" , "Common wild duck" , "Kind of duck" , "Ancestor of all domestic ducks" , "The famous steam train" .)