While UCS is an extremely useful command, it is not necessary in this case. All that is needed are the start point of the slice above the "invert" and the angle from the XY plane (25 deg) and the SLICE command.
The following assumes the start of the slice is 4" above the B.O.P., hence the "From" "QUA" "@0,0,4"
The second point on the slice only needs a vector of any magnitude, so I used @12<0<25. The 12 is the magnitude, the first angle "<0" is IN the XY plane and the second angle "<25" is FROM the XY plane.
The third point is just a vector relative to the second point, @22<90 would work, as would @0,12 or @ 0,1
Then just select the portion to keep.
SLICE
Select objects: 1 found
Select objects:
Specify first point on slicing plane by
[Object/Zaxis/View/XY/YZ/ZX/3points]
<3points>: From
Base point: Qua of <Offset>: @0,0,4
Specify second point on plane: @12<0<25
Specify third point on plane: @0,12
Specify a point on desired side of the plane or [keep Both sides]: B[/list:u]
Once you get familiar with 3D this kind of slice manipulation becomes second nature. For the neophyte, the UCS method is easier, if more steps are required.