Fatal weakness (8,4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fatal weakness</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for achilles heel that I've seen before include "Weak spot for attack" , "Vulnerable part" , "vulnerability" , "A chink in the armour" , "Small but fatal weakness" .)
(Other definitions for flaw that I've seen before include "Defect, imperfection" , "(Make) defect(ive)" , "Fault, blemish" , "weak spot" , "Imperfection or blemish" .)