Such a number could be written down too! (9)

I believe the answer is:

imaginary

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'such a number' is the definition.
(imaginary number is a kind of number)

I can't explain the remainder of the clue.

(Other definitions for imaginary that I've seen before include "Fictitious" , "Fictional" , "Pretend" , "Unreal, fanciful" , "Not real" .)

I've seen this clue in The Hindu.

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