Mark has day off cereal product (4)
I believe the answer is:
bran
'cereal product' is the definition.
(I've seen this in another clue)
I don't understand how the remainder of the clue works.
(Other definitions for bran that I've seen before include "Husks of corn" , "Element in high fibre diet" , "Type of roughage" , "Outer covering of grain" , "Horse food stored in barn" .)