Practically forever (3)

I think it could be either:

Which length description are you looking for?:

eon

'practically forever' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for eon that I've seen before include "Geological period" , "Immense period" , "In time, long division" , "Vast period" , "An immense time" .)

I've seen this clue in the Universal.

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