Tiny bit of cereal (5)
I believe the answer is:
grain
This clue is a double definition.
'tiny bit' is the first definition.
(I've seen this in another clue)
'cereal' is the second definition.
(I know that grain is a type of cereal)
'of' acts as a link.
(Other definitions for grain that I've seen before include "Small piece of sand" , "Eg, wheat" , "Enter into combat with" , "Small hard particle of anything" , "Smallest measure of weight in avoirdupois" .)