Foundation is corrupt (4)
I believe the answer is:
base
I believe this clue is a double definition.
'foundation' is the first definition.
(synonyms)
'corrupt' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for base that I've seen before include "Lowest part; ignoble" , "Not a noble" , "Not worth much" , "Vile; basis" , "Low and contemptible" .)