Perhaps Dennis bores by going on and on (8)

I believe the answer is:

timeless

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'on' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them 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 timeless that I've seen before include "Unaffected by fashion" , "Eternal or unconnected with the passing hours" , "Dateless" , "Immortal" , "Unchanged by age" .)

I've seen this clue in The Independent.

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