Age of misfortune? (8)

I believe the answer is:

thirteen

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'age of misfortune?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

This is all the clue.

Can you help me to learn more?

(Other definitions for thirteen that I've seen before include "Half of what's left [26D]" , "Where situation is" , "Cardinal" , "Some consider it unlucky" , "13" .)

I've seen this clue in the Evening Standard.

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