Defamation is the making of *false* factual statements to a third party, which damage your reputation and/or make others not want to work with you. If the first employer made false statements--for example, accused you of larceny when you did not in fact do that--then that may be defamation and you may therefore have a legal claim for compensation. On other hand, if you in fact did the thing you are accused of, that would not be defamation, since it is not defamation to make or report (and you can't sue over) true facts, even if the truth damages you. There is no right to stop people from speaking the truth about what you have done.