Mathematical feature in a made up edition (9,6)

I believe the answer is:

imaginary number

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'mathematical feature in' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps you can see an association between them that I don't see?

'a made up edition' is the wordplay.
'a made up' becomes 'imaginary' (I've seen this before).
'edition' becomes 'number' (I am not sure about this - if you are sure you should believe this answer much more).
'imaginary'+'number'='IMAGINARY NUMBER'

Can you help me to learn more?

(Another definition for imaginary number that I've seen is " In another case, I may get".)

I've seen this clue in The Times.

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