Top four for Arsenal, fantastic for periods of time (4)

I believe the answer is:

eras

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'periods of time' is the definition.
(an era is a period of time)

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

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

I've seen this clue in The Sunday Times and The Times.

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