Smallest possible quantity of cereal (5)
I believe the answer is:
grain
This clue might be a double definition.
'smallest possible quantity' is the first definition.
'grain' can be an answer for 'quantity' (grain is a kind of quantity). I am unsure of the 'smallest possible' bit.
'cereal' is the second definition.
(I know that grain is a type of cereal)
'of' is the link.
Can you help me to learn more?
(Other definitions for grain that I've seen before include "Silo contents" , "Enter into combat with" , "Small hard particle, of rice perhaps" , "Cereal crops" , "Cereal as food" .)