A person who clocks (illegally winds back the milometer of) a motor car(noun)
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Use "clocker" in a sentence
"The Six O'clocker's don't say nothing; it's the Eight O'clocker's always complaining."
"I like the early morning folks, the Six O'clocker's."
"The structure, alternating between Strike, the teenage clocker, and Rocco, the middle-aged detective, is unusually supple--Price doesn't give away all the information to set up a dramatic irony between the two, instead leaving the reader in the dark so that neither character becomes a dupe in our eyes."