Mark with two dots (5)

I believe the answer is:

colon

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

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

(Other definitions for colon that I've seen before include "Panama port; part of intestine" , "Part of the intestine - punctuation mark" , "Punctuate part of the large intestine" , "Punctuation mark; gut" , "Part of body; two dots" .)

I've seen this clue in the Evening Standard.

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