Nodding head at priestess before noon (8)

I believe the answer is:

nutation

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'nodding head at priestess before noon' is the definition.
I can't tell whether this definition defines the answer.

'head at priestess before noon' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'priestess before' becomes 'io' (I can't explain this - if you can you should give a lot more credence to this answer).
'noon' becomes 'n' (abbreviation).
'nut'+'at'+'io'+'n'='NUTATION'

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I've seen this clue in The Guardian.

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