Eating large, square meal, that's fine (5)
I believe the answer is:
flour
'meal that's fine' is the definition.
I can't judge whether this defines the answer.
'eating large square' is the wordplay.
'eating' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'square' becomes 'four' (four is a square number).
'l' going into 'four' is 'FLOUR'.
Can you help me to learn more?
(Other definitions for flour that I've seen before include "Ground and sifted cereal grain meal" , "Fine-ground grain" , "Miller's product" , "Powdered grain" , "Delivery from miller" .)