Deputy — failing (4)
I believe the answer is:
vice
This clue is a double definition.
'deputy' is the first definition.
(as in vice-president)
'failing' is the second definition.
(vice can mean a moral failing)
(Other definitions for vice that I've seen before include "Is the gripping tool evil?" , "venality?" , "perversion" , "Moral weakness" , "As a substitute for; immoral behaviour" .)