No, David Clarke did not plagiarize.

In a post How the media got it wrong on Sheriff Clarke plagiarism accusation, John Lott concludes
there was no plagiarism in the David Clarke matter:


By contrast, Clarke’s thesis footnoted on the same page each questionable sentence. It was sloppy of Clarke to copy five separate sentences in their entirety, but he did not commit plagiarism.



Elsewhere, there is mention of some Harvard plagiarism:



A number of Harvard law professors have faced plagiarism accusations. But in each and every case, then Dean Elena Kagan, now Supreme Court Justice, found that no rules had been broken.

Professor Alan Dershowitz was accused of copying verbatim 22 of the 52 endnotes in his book. At the end of the book, Dershowitz merely mentioned the book that he copied from.
Professor Laurence Tribe copied many passages almost word-for-word from another scholar’s work. He escaped blame because he had hired a first-year law student to ghostwrite the book for him. The student took the blame.
Professor Charles Ogletree used six consecutive paragraphs from another book. But it was decided that the mistake did not result from “deliberate wrongdoing.”



Previously, USAToday had a piece [ Report: Sheriff David Clarke plagiarized parts of homeland security thesis ] based on CNN which had noted:


“In all instances reviewed by CNN's KFILE, Clarke lifts language from sources and credits them with a footnote, but does not indicate with quotation marks that he is taking the words verbatim,” the story said.



With IPBiz defining plagiarism as "copying with attribution," this was not plagiarism. There was attribution at the point of the copying.

See also the May 31 post on IPBiz
Did Sheriff David Clarke copy without attribution?

See also Lawyers copying other lawyers: plus ça change, plus c'est la même chose