O my! —- missing from holy book is the nucleus (8)

I believe the answer is:

deuteron

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'nucleus' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Another definition for deuteron that I've seen is " Nucleus comprising a proton and a neutron".)

I've seen this clue in The Times.

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