Eternity (3)

I think it could be either:

Which length description are you looking for?:

eon

'eternity' 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 "Vast age (alt. sp.)" , "Geological period" , "billion years" , "thousand million years" , "Very long period (US and technical spelling)" .)

I've seen this clue in the Universal, The Telegraph and the Penny Press.

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