Warning of modern-day sin (6)
I believe the answer is:
advice
'warning' is the definition.
(thesaurus)
'modern-day sin' is the wordplay.
'modern day' becomes 'ad' (I've seen this in other clues).
'sin' becomes 'vice'.
'ad'+'vice'='ADVICE'
'of' acts as a link.
(Other definitions for advice that I've seen before include "Informed counsel" , "Guidance; delivery note" , "Opinion given about what to do" , "Counsel given" , "Supposed words of wisdom" .)