Element, one strengthening teeth – oddly easy to replace lack, likewise (7)

I believe the answer is:

caesium

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'element' is the definition.
(I know that caesium element symbol Cs)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for caesium that I've seen before include "Chemical element, symbol Cs" , "Metallic element with atomic number 55" , "Rare metal element, Cs" , "Reactive metal element" .)

I've seen this clue in The Guardian.

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