Often a clerk will file an answer with a request for entry of default pending and the result is that the answer trumps the request for default and the case just goes on without interruption. If that is what happened in your case, it is not clear why the opposing attorney would be pleading inadvertence--unless he had to file a Code of Civil Procedure section 473 motion to set aside the default.
Without seeing the papers filed, it is pretty difficult to answer your question how to meet the attorney's claim of inadvertence. A little more detail would help a lot. If you want to give me a call, I'll try to answer your question. Please have all the papers in the file handy when you call. (949) 455-0600. Good luck!
There seem to be quite a few details missing in your statement of facts, which makes it difficult to provide an adequate response.
However, if the Answer was filed prior to your Request for Default then the Court will likely deny your request. The rational is that a responsive pleading must be filed within the time specified in the summons OR within any further time that may be allowed. A plaintiff who has yet to request a default has, in effect, allowed the defendant further time to file a responsive pleading. So that's issue number one. Even though the Answer may not have been timely filed, if it was filed prior to your request for default then your request will probably be denied.
If your request for default was granted and a default judgment was entered against the Defendant, and now that Defendant is seeking to set aside the default on the grounds of his inadvertence then I would oppose that argument by showing the Defendant's actions and failure to act rise to a level beyond inadvertence and excusable neglect. I would argue that the Attorney's failure to adhere and obey a Court order is a dereliction of his duties as an attorney and rises to a level of UN-excusable neglect. I'd also show that the Summons & Complaint were properly served. That the Defendant failed to respond within the time stated int he Summons. Show that the Court provided a certain and specific day that the responsive pleading needed to be filed by and highlight the fact that the Defendant failed to adhere to the Court's order.
However, in the interests of justice and fairness Courts are often inclined to set aside default judgments and allow defendant's their day in court.
This response does not create an attorney-client relationship between you and I. I am not your lawyer and I am not representing you in the underlying issue stated in your question. The response I have offered is not intended to be relied upon, you should seek out an attorney to assist in this matter.
While I cannot give you legal advice in this forum, if the Answer is submitted to the clerk before the default is actually entered, I have never heard of the judge striking the answer and then entering the default. It does not appear to be an error. In my view, to have a chance at winning this type of motion, you need to cite some law that the clerk was "required" to enter the default despite the submission of the Answer. I am not sure if such law exists.
THIS RESPONSE IS INTENDED TO CONVEY GENERAL INFORMATION ONLY. IT SHOULD NOT BE RELIED UPON OR TAKEN AS LEGAL ADVICE. FURTHER, THIS RESPONSE IS NOT INTENDED TO AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP.