O being this kind of letter? (8)

I believe the answer is:

circular

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'letter?' is the definition.
(type of advertisement)

I cannot understand how the rest of the clue works.

(Other definitions for circular that I've seen before include "Round; general letter" , "Widely-distributed notice" , "Letter or notice copied to a number of people" , "plug, perhaps" , "Notice for wide distribution" .)

I've seen this clue in The Sun.

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