One’s own part arranged for spinet (6)

I believe the answer is:

instep

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'one's own' is the definition.
I can't tell whether this definition defines the answer.

'part arranged for spinet' is the wordplay.
'part' becomes 'in' (synonyms).
'arranged for spinet' becomes 'step' (I can't explain this - if you can you should believe this answer much more).
'in'+'step'='INSTEP'

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(Other definitions for instep that I've seen before include "Upper surface of the foot" , "Arched part of foot" , "One part of Oxford" , "Foot arch - conforming" , "Marching together correctly" .)

I've seen this clue in the Evening Standard.

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