A number eating large meal (5)
I believe the answer is:
flour
'meal' is the definition.
(flour is a kind of meal)
'a number eating large' is the wordplay.
'a number' becomes 'four' (four is a kind of number).
'eating' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'four' placed around 'l' is 'FLOUR'.
(Other definitions for flour that I've seen before include "Ground and sifted cereal grain meal" , "Wheat meal" , "Finely ground meal" , "Finely ground grain" , "Wheat powder" .)