Use heroin regularly for a long time (3)

I believe the answer is:

eon

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

I cannot understand how the rest of the clue works.

(Other definitions for eon that I've seen before include "Very long age" , "An immense time" , "Geological period" , "Vast period" , "billion years" .)

I've seen this clue in The Times.

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