Pumpernickel grain (3)
I believe the answer is:
rye
'pumpernickel grain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rye that I've seen before include "New York bread" , "old port" , "A Cinque Port; a grain" , "American whisky" , "Whiskey ingredient" .)