Foundations of army garrisons (5)
I believe the answer is:
bases
I believe this clue is a double definition.
'foundations' is the first definition.
('basis' can be a synonym of 'foundation')
'army garrisons' is the second definition.
(army bases)
'of' acts as a link.
(Other definitions for bases that I've seen before include "On which things stand" , "Supporting layers" , "Bottoms" , "Foundations" , "Fundaments" .)