Crossword Genius

Casino tycoon puts down $1000 on horse (5)

Ross

I believe the answer is:

pacer

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'horse' is the definition.
(a horse which paces)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for pacer that I've seen before include "early race leader?" , "One who sets the running speed; horse" , "One walking steadily" , "Competitor" , "walker?" .)

I've seen this clue in the Sydney Morning Herald.
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