Mark putting two together in a compound? (6)

I believe the answer is:

hyphen

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'mark' is the definition.
(hyphen is a kind of mark)

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

(Other definitions for hyphen that I've seen before include "Mark between parts of a compound word" , "A horizontal words-joiner" , "Word-joining mark" , "Dash breaking word" , "Horizontal words joiner" .)

I've seen this clue in The Hindu.

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