Confused as to cereal grains (4)
I believe the answer is:
oats
'cereal grains' is the definition.
(oat is a kind of cereal grain)
'confused as to' is the wordplay.
'confused' indicates anagramming the letters.
'as'+'to'='asto'
'asto' with letters rearranged gives 'OATS'.
(Other definitions for oats that I've seen before include "Porridge ingredients" , "Cereal grass and seeds" , "Wild ones are sown in youthful excess" , "Basis of porridge" , "Cereal food for horses" .)