Military installation (4)
I believe the answer is:
silo
'military installation' is the definition.
(silo is a kind of military installation)
This is all the clue.
(Other definitions for silo that I've seen before include "Tower for storing grain" , "Storage unit for animal feed" , "Cylindrical storage tower" , "Farm store; soil (anag.)" , "Storage space for fodder" .)