The sin of a substitute? (4)
I believe the answer is:
vice
I believe this clue is a double definition.
'the sin' is the first definition.
(thesaurus)
'a substitute?' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for vice that I've seen before include "Sin; weakness" , "person to stand in for skipper" , "may have a powerful grip" , "something antisocial" , "Gripper - in place of - immorality" .)