Foundation, bottom (4)
I believe the answer is:
base
This clue is a double definition.
'foundation' is the first definition.
(synonyms)
'bottom' is the second definition.
(I know that base is a type of bottom)
(Other definitions for base that I've seen before include "Low kind of foundation" , "Mean; HQ" , "Venial; headquarters" , "Mean - military installation" , "Ignoble; low part" .)