It's even handed to a mathematician (8)
I believe the answer is:
equation
'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" .)