A thousand million years (3)

I believe the answer is:

eon

'a thousand million years' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for eon that I've seen before include "Very long period (US and technical spelling)" , "Era" , "Long period of time" , "Very long age" , "Vast age; eternity" .)

I've seen this clue in the Universal, The Sun, The Times and The Sunday Times.

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