Here is one option. Create a new workplane using the ""on axis"" function. Build a closed profile on this workplane that includes the angled line that would enclose the portion of the existing tube to be removed. Use the punch function to cut away the angle.
Another option is to use a workplane on axis like Mike said, but then just move the face using the ""rotate"" option. When doing this, select the W axis of the workplane to rotate around.
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