It smoothly finishes off a piece of writing (6)

I believe the answer is:

screed

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'a piece of writing' is the definition.
(I know that screed is a type of piece of writing)

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

Can you help me to learn more?

(Other definitions for screed that I've seen before include "Tiresomely long letter about creeds" , "Tedious speech; cement" , "Lengthy statement" , "Layer of mortar to finish off a floor surface" , "Long tedious piece of writing" .)

I've seen this clue in The Globe and Mail.

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