Sign flat for example isn't wanted here (7)

I believe the answer is:

natural

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'sign flat' is the definition.
'natural' can be an answer for 'flat' (thesaurus). I am unsure of the 'sign' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for natural that I've seen before include "pure" , "one short of being sharp?" , "Additive-free" , "Not artificial or man-made" , "Innate - neither flat nor sharp" .)

I've seen this clue in The Irish Independent.

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