Within single part of shoe (6)
I believe the answer is:
insole
'part of shoe' is the definition.
(I know that insole is part of shoe)
'within single' is the wordplay.
'within' becomes 'in'.
'single' becomes 'sole' (synonyms).
'in'+'sole'='INSOLE'
(Other definitions for insole that I've seen before include "Padding in a shoe" , "Inner part of shoe" , "Oxford's content" , "support for rambler, perhaps" , "Part of a shoe or boot" .)