A dotty mark? (5)

I believe the answer is:

colon

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'mark?' is the definition.
(colon is a kind of punctuation mark)

I can't explain the rest of the clue.

(Other definitions for colon that I've seen before include "City at the Caribbean entrance to the Panama Canal" , "unit of foreign currency" , "that may cut sentence" , "Part of body; some punctuation" , "Punctuate part of the large intestine" .)

I've seen this clue in the Evening Standard.

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