An element of luminescence (6)

I believe the answer is:

radium

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'an element' is the definition.
(radium is a kind of element)

I can't explain the remainder of the clue.

(Other definitions for radium that I've seen before include "Curie-found element" , "Ra (atomic number 88)" , "Radioactive metal" , "Radioactive element discovered by the Curies" , "Radioactive metallic element found in pitchblende" .)

I've seen this clue in the Evening Standard.

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