Grain tower (4)
'grain tower' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for silo that I've seen before include "Grain storage tower" , "Grain storage area" , "Storage space for fodder" , "Tower for making fodder" , "Farm tower" .)
(Other definitions for barn that I've seen before include "Building for farm animals" , "Farm 4 down" , "Outlying farm building" , "Large shed" , "Farm store" .)