Insiders find only part of a boot (6)

I believe the answer is:

insole

I'm a little stuck... Click here to teach me more about this clue!

'part of a boot' is the definition.
(I know that insole is part of boot)

'insiders find only' is the wordplay.
'insiders find' becomes 'in' (I am not sure about this - if you are sure you should believe this answer much more).
'only' becomes 'sole' (synonyms).
'in'+'sole'='INSOLE'

Can you help me to learn more?

(Other definitions for insole that I've seen before include "Padding in a shoe" , "Inner part of shoe" , "Lesion in part of foot" , "Boots may be holding it" , "support for rambler, perhaps" .)

I've seen this clue in the Sydney Morning Herald.

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