Reconstructed barn for grain (4)
I believe the answer is:
bran
'grain' is the definition.
(I know that bran is a type of cereal)
'reconstructed barn' is the wordplay.
'reconstructed' indicates anagramming the letters.
'barn' anagrammed gives 'BRAN'.
'for' is the link.
(Other definitions for bran that I've seen before include "Type of edible fibre" , "Food from the husks of cereal grains" , "Husks of cereal seeds sifted from flour" , "Outer covering of grain" , "Separated grain husks" .)