The ornamental design for a portion of a displaypanel with an icon, as shown and described.
FIGURE is a front view of a portion of a display panel with an icon showing my new design.
The even-dash broken lines constituting a portion of the icon are unclaimed portions thereof and form no part of the claimed design. Dot-dash broken lines show portions of the display panel or screen, are boundaries of the claimed design and form no part of the claimed design.