Corrupt foundation (4)
I believe the answer is:
base
I believe this is a double definition.
'corrupt' is the first definition.
(thesaurus)
'foundation' is the second definition.
(synonyms)
(Other definitions for base that I've seen before include "Bottom support" , "Ignoble, military site" , "Interest charged by the Bank of England to commercial banks" , "unprincipled" , "Reprobate" .)