Sarah's and Cindy's definitions are good in this particular situation.
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In general though: "Clocked" is slang, and from what I can tell from etymology sites on-line, it originated in the UK (though I've heard it plenty here in the US).
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Originally it was used to refer to a car's odometer. When someone would roll-back the miles to misrepresent a vehicle and make it appear newer than it was, people would say that the car had been "clocked"...or more specifically that the odometer had been "clocked." (refering to the rolling back of time)
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So essentially, "clocked" refers to a situation where it becomes apparent that someone/something has been falsified/faked to appear as something it is not.
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Given that origin though, I do wonder why as community we use the term, as it tends to reinforce the false idea that we aren't being genuine. I guess I'm a bit more partial to the slang "read."