Chemical element, if contracted, could be not available (6)

I believe the answer is:

sodium

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'chemical element' is the definition.
(sodium is a kind of chemical element)

I can't explain the rest of the clue.

(Other definitions for sodium that I've seen before include "The Na in NaCl" , "Common salt is ...... chloride" , "Element discovered by Humphry Davy" , "Element in common salt" , "Atomic number 11" .)

I've seen this clue in The Guardian.

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