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' indicates a hidden word.
'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" .)