2ways:
1) Place a roof below the wall at the requested slope. Use the Trim to Roof command, Trim Base. You can delete the roof afterwards, or hide its layer. The cut will stay fixed until you click the Undo Roof Trim button in the wall's settings dialog.
2) Place a roof below the wall at the requested slope. Do a Solid Element Operation (SEO) , downwards extrusion, with the roof as operator and the wall as target. Afterwards, hide the operators layer. The cut will stay but move with the relative movements of the wall vs. the operator. If you delete the operator the cut will disappear.
And: You'd get this answer a lot faster if you had bothered to do a Search in this forum before posting your question.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1