Type of grain (3)
I believe the answer is:
rye
'type of grain' is the definition.
(I know that rye is a type of caryopsis)
This is all the clue.
(Other definitions for rye that I've seen before include "old port" , "J D Salinger novel 'The Catcher in the ...'" , "E. Sussex town; cereal" , "Grass related to wheat" , "Hardy cereal crop" .)