Bottom, foundation (4)
I believe the answer is:
base
This clue is a double definition.
'bottom' is the first definition.
(base is a kind of bottom)
'foundation' is the second definition.
(synonyms)
(Other definitions for base that I've seen before include "Mean; HQ" , "Pedestal" , "Plinth" , "Lowest part; ignoble" , "Ignoble; starting point" .)