Failing to become a deputy (4)
I believe the answer is:
vice
This clue is a double definition.
'failing' is the first definition.
(vice can mean a moral failing)
'a deputy' is the second definition.
(as in vice-president)
'to become' is the link.
(Other definitions for vice that I've seen before include "Sin; weakness" , "Moral weakness" , "Prefix for deputy to, say, chairman" , "Gripper - in place of - immorality" , "Is the gripping tool evil?" .)