Crossword Genius

Age’s back-page planned (3)

Ross

I believe the answer is:

eon

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'age's' is the definition.
(eon is a kind of age)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for eon that I've seen before include "billion years" , "Era" , "Immense period" , "Vast period" , "thousand million years" .)

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