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Cheered when it tried to get rid of its rider (6)

Ross

I believe the answer is:

bucked

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'cheered' is the definition.
Although both the answer and definition are past participle verbs, I don't understand how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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