Warning of today's sinfulness? (6)
I believe the answer is:
advice
'warning' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for advice that I've seen before include "Counsel given" , "Guidance; delivery note" , "'Counsel, guidance (6)'" , "Opinion offered as to future action" , "tip" .)