Told to mend part of sock (4)
I believe the answer is:
heel
'part of sock' is the definition.
(heel is a part of a sock)
'told to mend' is the wordplay.
'told' shows a homophone (sound like).
'to mend' becomes 'heal' ('heal' can be a synonym of 'mend').
'heal' sounds like 'HEEL'.
(Other definitions for heel that I've seen before include "One of the crusty ends of a loaf" , "order to dog" , "villain" , "Achilles's weak spot" , "3's weak point" .)