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