Certain as Pygmalion writer's sometimes said to be (4)

I believe the answer is:

sure

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'certain' is the definition.
(I know that certain can be written as sure)

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

Can you help me to learn more?

(Other definitions for sure that I've seen before include "Completely certain" , "Agreed" , "Certain, confident" , "of course" , "Undoubtedly true" .)

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