Wooden timbers said to be an essential element (4)

I believe the answer is:

crux

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

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for crux that I've seen before include "Main point (of the matter)" , "Nub" , "Key point" , "Most important aspect" , "Particular difficulty" .)

I've seen this clue in The Times.

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