Yes, clashing with police at a KKK riot does make Fred Trump a Kluxer. Like millions of other people.
The KKK wasn't all about black people at the time. It was as much an anti-Catholic and anti-Jewish movement in the 1920's. Millions of Americans were sure that all the Poles, Jews, Italians, Irish who'd been entering the US for decades were unassimilable groups who could never be Americanized.
In 1927, Donald Trump’s father was arrested after a Klan riot in Queens
By Philip Bump The Washington Post February 29 at 2:06 PM

Klan members march through Queens in May 1927. (Brooklyn Daily Eagle) ____________________________________________
................ On Memorial Day 1927, brawls erupted in New York led by sympathizers of the Italian fascist movement and the Ku Klux Klan. In the fascist brawl, which took place in the Bronx, two Italian men were killed by anti-fascists. In Queens, 1,000 white-robed Klansmen marched through the Jamaica neighborhood, eventually spurring an all-out brawl in which seven men were arrested.
One of those arrested was Fred Trump of 175-24 Devonshire Rd. in Jamaica.

This is Donald Trump's father. Trump had a brother named Fred, but he wasn't born until more than a decade later. The Fred Trump at Devonshire Road was the Fred C. Trump who lived there with his mother, according to the 1930 Census.

The predication for the Klan to march, according to a flier passed around Jamaica beforehand, was that "Native-born Protestant Americans" were being "assaulted by Roman Catholic police of New York City." "Liberty and Democracy have been trampled upon," it continued, "when native-born Protestant Americans dare to organize to protect one flag, the American flag; one school, the public school; and one language, the English language."
It's not clear from the context what role Fred Trump played in the brawl. The news article simply notes that seven men were arrested in the "near-riot of the parade," all of whom were represented by the same lawyers. Update: A contemporaneous article from the Daily Star notes that Trump was detained "on a charge of refusing to disperse from a parade when ordered to do so." ................ An article in the Brooklyn Daily Eagle from that December notes that the Klan would probably weigh in heavily against the potential presidential nomination of then-New York Gov. Al Smith, given that he was a Catholic and a "champion of 'alienism.'" .............
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