How a mortise and tenon can be considered (7)

I believe the answer is:

jointly

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'how' is the definition.
Both the answer and definition are adverbs. Maybe there's an association between them I don't understand?

I don't understand how the remainder of the clue works.

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(Other definitions for jointly that I've seen before include "Collectively" , "Together" .)

I've seen this clue in the Evening Standard.

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