Half of those on bus superannuated (5)

I believe the answer is:

passe

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'half' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for passe that I've seen before include "dated" , "Past its prime - in France" , "Old hat" , "Outdated" , "-- -partout" .)

I've seen this clue in The Guardian.

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