It isn't corruption - it is corruption (5)
I believe the answer is:
taint
'it isn't' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for taint that I've seen before include "Stigmatise" , "Defile" , "Source of infection" , "contaminant" , "Defect - contamination" .)