Failing; deficiency (11)
I believe the answer is:
shortcoming
I believe this clue is a double definition.
'failing' is the first definition.
(I have seen 'A failing ' mean 'shortcoming' so perhaps 'failing' could also mean 'shortcoming')
'deficiency' is the second definition.
(I've seen this in another clue)
(Other definitions for shortcoming that I've seen before include "Weakness" , "A failing" , "drawback" , "Fault, defect" .)