Corrupt at the lowest level (4)
I believe the answer is:
base
I believe this clue is a double definition.
'corrupt' is the first definition.
(thesaurus)
'the lowest level' is the second definition.
(thesaurus)
'at' is the link.
(Other definitions for base that I've seen before include "Ignoble, military site" , "unprincipled" , "Mean - military installation" , "Found - worthless" , "Lowest part of something" .)