The part of 'Achilles'? (4)
I believe the answer is:
heel
'the part' is the definition.
(heel is a kind of part)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for heel that I've seen before include "Not the sole - scoundrel" , "Part of shoe; bad person" , "Despicable type" , "afoot?" , "order to dog" .)