Can I file a case for defamation if I am being falsely accused at work?

Defamation is legally defined as the "holding up of a person to ridicule, scorn or contempt in a respectable and considerable part of the community to the damage of the reputation of another." Since his opinion is only limited to an audience of one (or perhaps all the officers within the corporation) it cannot be said that the statements are to a considerable part of the community, so the statements do no legally qualify as defamatory, without considering whether the statements are meant to ridicule, scorn or lower your reputation in the community.

yes, you may sue any one for any thing, even a frivolous or false reason. No one will stop you from filing a suit. But will you win. Likely not. You will be required to prove actual damages, no speculative damages, no wounded feelings, etc. The law does not address every wrong that we may feel in life.

Defamation claims in employment typically are a worker's compensation claim.

Have you been damaged? Is it a lie? Can you take care of it by explaining the situation? Does the person you want to sue have enough money to make it worthwhile? A lot of people think immediately of suing and it is not always the right answer. Perhaps you need to explain to the person making the complaint why he is wrong.