I haven’t used FreeCAD before, but if it’s anything like inventor or fusion360, you might be able to edit the individual part and either reload that one part into the assembly or start a new assembly. Do you still have the .ipt file or equivalent for that one piece?

I’ve used Inventor and Fusion360 for small personal projects since 2014. In an instance like this, I would just draw a quick 2d shape off of the front face in your picture and cut it. Sometimes it’s just not worth the time to figure out. Are you modeling professionally?