One's own part arranged for spinet (6)

I believe the answer is:

instep

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'one's' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'own part arranged for spinet' is the wordplay.
'spinet' anagrams to 'INSTEP'.
However, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(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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