It's not safe to start riding on something high (5)

I believe the answer is:

risky

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'high' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for risky that I've seen before include "Hazardous" , "Treacherous" , "taking a chance" , "admitting possibility of unpleasant outcome" , "Potentially dangerous" .)

I've seen this clue in the Evening Standard.

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