Crossword Genius

No end of room for horses (4)

Ross

I believe the answer is:

stud

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But I can't explain it at all! Could you help me understand?

(Other definitions for stud that I've seen before include "card game" , "Fastening for collar" , "Virile man" , "Decoration" , "Ladies' man?" .)

I've seen this clue in The Telegraph.
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