Grain: grind it, I say (6)
I believe the answer is:
millet
'grain' is the definition.
(I know that millet is a type of cereal)
'grind it i say' is the wordplay.
'grind' becomes 'mill' (milling is a kind of grinding).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'i say' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'mill'+'e'+'t'='MILLET'
(Other definitions for millet that I've seen before include "The grain was ill-met, oddly" , "Seed from cereal grasses - French painter, d.1875" , "Cereal - French painter, d. 1875" , "sort of grass" , "Type of cereal" .)