Use "Slice by Workplane", a very handy tool.
On a tube you cant set the workplane with controll so go to Front View and set workplane by aktive view, then Draw a Box that follow the place you want to to slice, then set workplane by facet on the Box.
Activate Slice tool, check for "by Worplane" and also "Delete the sliced" part is good to use, then click on the object and a second time on the part that you want to take away, finish
V18, V19, V20, V21, 2015/ 16/ 17 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB