I’m angry, holding on for a significant period (4,3)

I believe the answer is:

iron age

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'period' is the definition.
(prehistoric period)

I don't understand the rest of the clue.

(Other definitions for iron age that I've seen before include "It followed the Bronze Age" , "Time of the spread of metal weapons, tools etc" , "historical period" , "Archaeological period" , "Period from about 1100 BC" .)

I've seen this clue in The Times.

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