In order to claim for defamation, you need to show 3 things:
Publication – That someone else (a natural person, company or incorporated association) communicated information to another person or other people (other than yourself);
Identification – The information identifies you or makes it clear to others that the communication is about you; and
Defamatory Content – The information is defamatory.
Has the company's accusation of fraud been made to anyone other than yourself? If it has not been published - to people other than yourself then you cannot establish defamation