Basically, the House Committee's hearings made plain that many departments of government and media organizations were riddled with Communists and Communist sympathizers. They hide and desembled, but they were proven to be there, both then and by the evidence uncovered after the fall of the Soviet Bloc.
McCarthy, who was an extremely popular Air Veteren politician, over reached subsiquently and his personal failings were used as a strawman to make it appear that the vast communist exposures were a fiction.
In truth however, the communists and their agents and proxies were actually guilty as charged in the vast majority of cases and only a small percentage were uncovered then and the Verona Files from the fallen Soviet bloc make that clear.
Lattimore was a vocal advocate for the Communists replacing Chiang Kai-Shek and the former Chinese ambassador testified the Communist revolution was greatly aided by Lattimore, Acheson, Service, et al.
Lattimore publicly advocated that Britain and the U.S. withdraw their influence from Asia and allow the Soviets to expand their influence there. He said communism would bring stability and industrialization to China.
He had close, long-standing associations with numerous people whose names appeared in the Venona papers but his name wasn't found. There is a long FBI file on Lattimore at http://foia.fbi.gov/owenlatt/owenlatt1a.pdf
It's certainly correct to say that Lattimore was one of the most outspoken advocates for Mao's gang to take over China.
Not prosecuted, yes.
Cleared, no.
Basically, Lattimore didn't need or want to be paid by the Soviets for what he believed in.
Also, a longtime good buddy of the Commie VP Wallace that I mentioned earlier.