Sore; corrupting influence (5)
I believe the answer is:
ulcer
I believe this is a double definition.
'sore' is the first definition.
(I've seen this before)
'corrupting influence' is the second definition.
(ulcer can mean a moral blemish)
(Other definitions for ulcer that I've seen before include "Cruel kind of open sore" , "Inflamed lesion" , "Spot of bother" , "Continuing source of pain" , "Cruel kind of bodily sore" .)