Period of sinister activity (3)

I believe the answer is:

era

'period' is the definition.
(era is a kind of period)

'of sinister activity' is the wordplay.
'of' indicates a hidden word.
'ERA' is hidden in the letters of 'sinister activity'.

(Other definitions for era that I've seen before include "Eon" , "Significant time" , "Memorable period" , "Series of years" , "Age of importance" .)

I've seen this clue in the Evening Standard.

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