Barn used for grain (4)
I believe the answer is:
bran
'for grain' is the definition.
(thesaurus)
'barn used' is the wordplay.
'used' is an anagram indicator.
'barn' with letters rearranged gives 'BRAN'.
(Other definitions for bran that I've seen before include "Separated grain husks" , "Milled grain husk" , "Type of edible fibre" , "Husks valued as fibre" , "Healthy breakfast" .)