Crossword Genius

It's fractionally down (11)

Ross

I believe the answer is:

denominator

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

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

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for denominator that I've seen before include "Lower number" , "Divisor" , "what's under the line?" , "Lower half of fraction (could be common!)" .)

I've seen this clue in The Globe and Mail.
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