Crossword Genius

Age of bird, not beast (3)

Ross

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)

'bird not beast' is the wordplay.
I cannot quite see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
The remaining letters 'en' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'of' acts as a link.

Can you help me to learn more?

(Other definitions for eon that I've seen before include "Huge time period" , "In time, long division" , "Major division of geological time" , "Age, epoch" , "Era" .)

I've seen this clue in The Guardian.
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