Grain used as ingredient of goat stew (4)
I believe the answer is:
oats
'grain' is the definition.
(I know that oats is a cereal)
'ingredient of goat stew' is the wordplay.
'ingredient of' indicates a hidden word.
'OATS' is hidden in the letters of 'goat stew'.
'used as' is the link.
(Other definitions for oats that I've seen before include "Seed grasses" , "Grains used as food and fodder" , "Cereal plants" , "Porridge ingredients" , "Edible grass seeds" .)