The campaign worker was a volunteer, who was immediately fired. Clinton had no role in making the initial claim. The worker was fired and that would have been the end of the story. Instead, Trump made it an issue for years.
He is well known for making crazy claims with no facts to back them up. He recently tweeted:
"Animals representing Hillary Clinton and Dems in North Carolina just firebombed our office in Orange County because we are winning"
Problem is that his ardent supporters believe this crap.