Dear Wanda: What you're describing may or may not constitute defamation, depending on a number of factors. I discuss the elements of defamation on my blog. As you will see, key facts will include things like (a) what, exactly, was said, (b) whether the accusation was made to you, to a police officer, or some uninterested third party; and (c) whether the storekeeper had good cause for believing that you did, in fact, steal merchandise.
NOTE: This communication is not intended as and should not be interpreted as legal advice. Rather, it is intended solely as a general discussion of legal principles. You should not rely on or take action based on this communication without first presenting ALL relevant details to a competent attorney in your jurisdiction and then receiving the attorney's individualized advice for you. This post does not create any attorney-client relationship and is not protected by the attorney-client privilege.