Greek character's mark of disgrace, with time passing (5)
I believe the answer is:
sigma
'greek character's' is the definition.
(letter from the Greek alphabet)
I cannot understand how the remainder of the clue works.
(Other definitions for sigma that I've seen before include "character in Athens" , "Letter from Athens" , "Greek letter signifying sum in maths" , "Athenian character" , "local opening in Salonika" .)