Menu's starter - a large fat duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(I know that mallard is a type of duck)
'menu's starter a large fat' is the wordplay.
'starter' suggests taking the first letters.
'large' becomes 'L' (eg in clothes sizes).
'fat' becomes 'lard' (lard is a kind of fat).
The first letter of 'menus' is 'm'.
'm'+'a'+'l'+'lard'='MALLARD'
(Other definitions for mallard that I've seen before include "Ancestor of all domestic ducks" , "Kind of duck" , "Oldest breed of duck" , "The famous steam train" , "Type of duck - world speed record holding steam engine (1938)" .)