Deficiency or failing (8)
I believe the answer is:
weakness
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">deficiency</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">failing</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(failing can mean a weakness or flaw)</span><br/ ><br/ ></span>
(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" .)