Use the Fillet and Trim function to create a trimmed fillet arc.
Using the Function:
Select Trim from the Modify>Fillet submenu.
Enter a value for the radius of the arc fillet.
Select the first fillet entity. (You can click anywhere on the entity; you do not have to select any specific location relative to the arc radius.)
Select the second curve on the portion of the curve that you want to remain. The selected entities will be trimmed, excluding closed curves, to the intersections with the arc fillet.
Above > An example of Fillet and Trim