Morsel a horse'll eat (3)
I believe the answer is:
oat
'morsel a horse'll eat' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oat that I've seen before include "Cereal plant" , "Cereal (singular)" , "Basis of porridge" , "Cereal grass" , "Food grain" .)