There is no such plea in the US. There may, in some courts, be a plea of "guilty, with an explanation" or a judge in a particular court may choose to recognize one and allow the defendant to enter evidence after a guilty plea that acts to mitigate or excuse their guilt -- but they have still admitted that they broke the law, and pled guilty. I've seen plenty of judges listen to the start of the "explanation" and change the plea to "not guilty."but that's plead, NOT GUILTY WITH AN AFIMATIVE defence, not, just plead with an affirmative defence as you said
A plea of Not Guilty doesn't necessarily say "I didn't do it." It says, plainly, "prove it!" The prosecution, within the US, has the burden of showing beyond a reasonable doubt that a defendant who pleads not guilty did actually commit the alleged act. The defendant has no burden to prove anything or enter a single item of evidence or word of testimony.