Grain of loathing (3)
I believe the answer is:
oat
'grain' is the definition.
(I know that oat is a type of cereal)
'of loathing' is the wordplay.
'of' says the answer is hidden in the clue.
'OAT' can be found hidden inside 'loathing'.
(Other definitions for oat that I've seen before include "Cereal grass" , "Basis of porridge" , "Cereal plant" , "Cereal (singular)" , "Food grain" .)