It's fractionally down (11)
I believe the answer is:
denominator
'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!)" .)