Crossword Genius

There are two sides to every one (8)

Ross

I believe the answer is:

equation

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for equation that I've seen before include "Maths statement with =" , "Ambrose --, author of The Devil's Dictionary" , "It can be algebraic" , "Situation where many factors need consideration" , "Mathematical statement that two expressions are equal" .)

I've seen this clue in the Evening Standard.
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