Kind of sin or virtue — or expert on the difference? (8)
I believe the answer is:
cardinal
'difference?' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cardinal that I've seen before include "Leading RC dignitary" , "Most important number" , "Main point of compass - most important" , "Scarlet; fundamental" , "High-ranking official of the Roman Catholic church" .)