The ex is no longer in her prime (3)

I believe the answer is:

old

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'prime' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for old that I've seen before include "out of the ark" , "Senescent" , "Obsolete" , "Long-lived" , "Wild clematis" .)

I've seen this clue in the Evening Standard.

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