I am so put out about Special K (9)

I believe the answer is:

potassium

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'k' is the definition.
('potassium' can be a synonym of 'k')

I can't explain the remainder of the clue.

(Other definitions for potassium that I've seen before include "Silvery metallic element" , "Soft metal, symbol K" , "Element K" , "Soft reactive metal" , "Soft metallic element" .)

I've seen this clue in The Independent.

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