Old times usually great (4)

I believe the answer is:

eras

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'old times' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for eras that I've seen before include "Epochs" , "At least two times" , "Long and distinct periods of history" , "typeface" , "Periods, ages" .)

I've seen this clue in the Evening Standard.

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