Grain used to make kvass (3)

I believe the answer is:

rye

(Other definitions for rye that I've seen before include "A Cinque Port; a grain" , "Hardy annual cereal grass" , "Grass - whiskey" , "J D Salinger novel 'The Catcher in the ...'" , "Hardy cereal crop" .)

I've seen this clue in the Universal.

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