Crossword Genius

Go on at a horse (3)

Ross

I believe the answer is:

nag

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'horse' is the definition.
(term for an old horse)

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

Can you help me to learn more?

(Other definitions for nag that I've seen before include "Old Arab maybe" , "Annoy constantly" , "Annoy the old horse persistently" , "Constant worrier" , "Equine" .)

I've seen this clue in The Mirror.
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