Stout grain (4)
I believe the answer is:
malt
'stout grain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for malt that I've seen before include "Grain product used in beer and whisky" , "Grain prepared for brewing or distilling" , "Brewing ingredient derived from grain" , "Pre-soaked kiln-dried cereal grain" , "Brewing agent" .)