One terribly long time! (3)

I believe the answer is:

eon

'long time' is the definition.
(eon is a kind of long time)

'one terribly' is the wordplay.
'terribly' is an anagram indicator.
'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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