Cheerios grains (4)
I believe the answer is:
oats
'cheerios grains' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oats that I've seen before include "Porridge ingredient" , "Porridge mix" , "Hardy cereals eaten by horses" , "Porridge ingredients" , "Cereal food for horses" .)