Crossword Genius

Mark, caught by rebounding bullets, say (5)

Ross

I believe the answer is:

comma

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'mark' is the definition.
(punctuation mark)

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

Can you help me to learn more?

(Other definitions for comma that I've seen before include "shortest interval" , ", (stet)" , "I signal a break" , "Punctuation mark - butterfly" , "Kind of butterfly" .)

I've seen this clue in The Independent.
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