Mules chewed one's cereal (6)
I believe the answer is:
muesli
'cereal' is the definition.
(I know that muesli is a type of cereal)
'mules chewed one's' is the wordplay.
'chewed' indicates an anagram.
'one' becomes 'I' (Roman numeral).
'mules' anagrammed gives 'muesl'.
'muesl'+'i'='MUESLI'
(Other definitions for muesli that I've seen before include "healthy breakfast?" , "Swiss cereal" , "eaten at breakfast" , "Mixture of cereals etc" , "Breakfast food of cereal, fruits and nuts" .)