Relax, I'll arnica parts of the armpit (7)

I believe the answer is:

axillar

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'the armpit' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?

'relax i'll arnica parts' is the wordplay.
'parts' indicates a hidden word (part of the clue provides the answer).
'AXILLAR' is hidden in the letters of 'relax ill arnica'.

'of' acts as a link.

Can you help me to learn more?

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

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