Age of the Odeon (3)

I believe the answer is:

eon

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

'age' is the definition.
(I know that eon is a type of age)

'the odeon' is the wordplay.
'EON' is hidden in amongst the letters.
But, I'm uncertain how this is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for eon that I've seen before include "Geological period" , "Very long period (US and technical spelling)" , "Very long age" , "Era" , "In time, long division" .)

I've seen this clue in the Evening Standard.

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