Yesteryear's traitor not popular (6)

I believe the answer is:

former

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'popular' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

I can't explain the rest of the clue.

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(Other definitions for former that I've seen before include "One-time" , "The first mention of the two" , "The first of two mentioned" , "Past; first (of two)" , "Previous" .)

I've seen this clue in The Guardian.

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