Reuben bread (3)
I believe the answer is:
rye
'reuben bread' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rye that I've seen before include "Cereal grass - Sussex Cinque Port" , "New York bread" , "Grass; whisky" , "21 [GRASS]" , "J D Salinger novel 'The Catcher in the ...'" .)