Back part of a sock (4)
I believe the answer is:
heel
'back part of a sock' 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 "Part of shoe; bad person" , "One of the crusty ends of a loaf" , "secure ball" , "As a dog, obey" , "scumbag" .)