Description of an eternity ring? (5-6)

I think it could be either:

Which length description are you looking for?:

5-6

endless

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If this is the answer, I'm afraid I cannot explain it.

(Other definitions for endless that I've seen before include "Unceasing" , "permanent" , "Tiresomely long" , "Continuous - eternal - incessant" , "Constant" .)

I've seen this clue in The Telegraph and The Sun.

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