Rigour requires twine to have 40% efficiency (10)

I believe the answer is:

stringency

I'm a little stuck... Click here to teach me more about this clue!

'rigour' is the definition.
(I've seen this before)

'twine to have 40% efficiency' is the wordplay.
I cannot really understand how this works, but
'twine' could be 'string' (synonyms) and 'string' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'requires' is the link.

Can you help me to learn more?

(Another definition for stringency that I've seen is " Rigour".)

I've seen this clue in the Sydney Morning Herald.

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