Barley used in brewing (4)
I believe the answer is:
malt
'barley used in brewing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for malt that I've seen before include "Cereal grain for distilling" , "Brewing ingredient derived from grain" , "strong drink" , "Barley product used in brewing" , "Ingredient of beer" .)