A foundation is corrupt (5)
I believe the answer is:
abase
'is corrupt' is the definition.
(thesaurus)
'a foundation' is the wordplay.
'foundation' becomes 'base' (synonyms).
'a'+'base'='ABASE'
(Other definitions for abase that I've seen before include "put to shame" , "Lower" , "what 11 [REPRIMANDS] are meant to do" , "Degrade (oneself)" , "Humiliate" .)