Crossword Genius

It's even handed to a mathematician (8)

Ross

I believe the answer is:

equation

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

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for equation that I've seen before include "feature of maths" , "Mathematical statement using the = sign" , "Mathematical statement of equivalence" , "E=mc2, say" , "See 5" .)

I've seen this clue in the Evening Standard.
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