Bit in a horse's mouth (3)
I believe the answer is:
oat
'bit in a horse's mouth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oat that I've seen before include "Cereal grass" , "Basis of porridge" , "Food grain" , "Cereal (singular)" , "Cereal plant" .)