Wooden timbers said to be an essential element (4)
I believe the answer is:
crux
'element' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand how the rest of the clue works.
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" .)