Crossword Genius

Essential cosine unequal and openly wrong (4,3,3)

Ross

I believe the answer is:

sine qua non

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

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

'cosine unequal and openly wrong' is the wordplay.
I cannot quite see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'wrong' could be 'no' ('no' can mean what just someone said is wrong) and 'no' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for sine qua non that I've seen before include "Essential condition" , "Must" , "A vital qualification" , "Indispensable condition" .)

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