Achilles' weak point
I believe the answer is:
heel
'achilles' weak point' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for heel that I've seen before include "Achilles's weak spot" , "See 21" , "bank" , "Slang scoundrel or rear of foot" , "Crusty end of a loaf" .)