Achilles __ (weak spot) (4)
I believe the answer is:
heel
'achilles' weak spot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for heel that I've seen before include "despicable person" , "tip" , "Slang scoundrel or rear of foot" , "See 1 down" , "swine" .)