Barn designed for grain (4)
I believe the answer is:
bran
'grain' is the definition.
(thesaurus)
'barn designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'barn' is an anagram of 'BRAN'.
'for' is the link.
(Other definitions for bran that I've seen before include "Husks of cereal grain" , "Food from the husks of cereal grains" , "Food for horses in barn" , "Husks of corn" , "Dietary fibre source" .)