Trying to devise a one-size-fits-all description for ultraprocessed foods is flummoxing federal regulators. For months, the health secretary, Robert F. Kennedy Jr., has promised to create a definition of ultraprocessed, a crucial part of the Make America Healthy Again agenda. In mid-April, he testified before Congress that the Food and Drug Administration had forwarded a definition to other agencies, including the Department of Agriculture. But behind the scenes, officials said, the process of defining ultraprocessed foods is still very much up in the air. Agencies are struggling to agree, and it is unclear when a definition will be released. “It’s not final until it’s final,” said Calley Means, a senior adviser to Kennedy, adding that the definition would ultimately be the result of hundreds of conversations with scientists, agency staff and other stakeholders.…