Liner in Korean city's sound (6)
I believe the answer is:
'liner' is the definition.
'in korean city's sound' is the wordplay.
'korean city's sound' becomes 'sole' (I can't justify this - if you can you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for insole that I've seen before include "Part of Oxford perhaps" , "Lesion in part of foot" , "Part of a shoe or boot" , "Boots may be holding it" , "Padding in a shoe" .)