My husband''s ex-wife filed to get her child support through the state of MD. He has never missed a payment, is this defamation of character?

In order to support a claim for defamation, where the statute of limitation is one year, you must have damages. This is very speculative.  If for some reason, your hunch comes to fruition, you could file a civil claim in either small claims or regular court but it sounds like you are worrying unnecessarily.  Furthermore, this type of issue, mainly where, spouses accuse each other of false and unsavory dealings, is prevalent.  Unfortunately, this perception will likely dilute the strength of a claim.  This all will come down to severity.