One terribly long time! (3)

I believe the answer is:

eon

'long time' is the definition.
(I've seen this before)

'one terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'one' anagrammed gives 'EON'.

(Other definitions for eon that I've seen before include "Prolonged period" , "Vast age (alt. sp.)" , "Very long period (US and technical spelling)" , "Age, epoch" , "Geological period" .)

I've seen this clue in the Evening Standard.

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