Did Adam have one? (5)

I believe the answer is:

navel

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'one?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for navel that I've seen before include "Umbilicus" , "Something to contemplate" , "Tummy button" , "Bellybutton" , "Central point" .)

I've seen this clue in the Evening Standard.

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