Achilles' heel (4,4)
I believe the answer is:
weak spot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">achilles heel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for weak spot that I've seen before include "Point of vulnerability" , "Achilles heel?" , "Area in which one is not knowledgeable" , "Gap in knowledge" , "'Your area of vulnerability (4,4)'" .)