Deficiency or failing (8)
I believe the answer is:
weakness
I believe this clue is a double definition.
'deficiency' is the first definition.
(thesaurus)
'failing' is the second definition.
(failing can mean a weakness or flaw)
(Other definitions for weakness that I've seen before include "Ill-defended spot - predilection" , "Fault or flaw" , "Lack of strength" , "Failing or shortcoming" , "Achilles heel" .)