Soundly cure part of foot (4)
I believe the answer is:
heel
'part of foot' is the definition.
(heel is a part of the foot)
'soundly cure' is the wordplay.
'soundly' shows a homophone (sound like).
'cure' becomes 'heal' (synonyms).
'heal' sounds like 'HEEL'.
(Other definitions for heel that I've seen before include "American rotter or shoe part" , "Untrustworthy person" , "Rogue" , "Keel over" , "A cad and a bounder!" .)