There are two sides to every one (8)
I believe the answer is:
equation
'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" .)