If you want to be alert when a desired callsign appears in cluster, add this command (Linux) in Preferences -> DXCluster.
You should install zenity first.
zenity --info --text='Callsign $CALLSIGN detected at $FREQ $MODE' --title=Info
Also you may set display duration in 5 seconds (Thanks DL2KI). After the window closes automatically.
kdialog --title=Spot-Info --passivepopup "Callsign $CALLSIGN detected at $FREQ $MODE" 5
This is also possible with "notify-send". The display duration is set in ms (-t 5000).
notify-send -t 5000 "Spot-Info" "Callsign $CALLSIGN detected at $FREQ $MODE"