Crossword Genius

Additional horsemen? (6)

Ross

I believe the answer is:

riders

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'horsemen?' is the definition.
(horseman is a kind of rider)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for riders that I've seen before include "they mount up" , "They sit" , "Equestrians" , "Knights" , "Clauses appended to legislative bills" .)

I've seen this clue in The Globe and Mail.
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