Defamation is when someone makes a false statement (oral or written) about you, to someone other than you, that causes you harm. If the harm is to your professional reputation, it's defamation per se. Defamation in the employment context can be tricky becuase employers can comment about performance issues and give opinions without fear of defamation claims. However, if they make false factual statements, you may have a defamation claim.
You should find an attorney that specializes in employment law.
The "at will" doctrine is often a red herring that some employees swallow. If you are terminated for an "unlawful" reason, you have a cause for wrongful termination. If the "conflict of interest" reason was false and a true motivation such as discrimination based on for example age or race is proved, the the termination is unlawful. Many employment lawsuits seek additional causes of action (theories of your case) such as breach of contract (if the employer violated a contract or written policies) and defamation in a proper case. So much defends on the particulars of your specific case, and you may consult with an attorney experienced in litigating employment matters. Many offer a free, confidential and no obligation consultation. The remedies you seek may vary depending on your situation, but it does not hurt to ask and then proceed in the manner most appropriate for your goals. Litigation or negotiation may yield reinstatement, monetary damages and send a strong message to employers who stray from the dictates of the law.