Make a copy of your filtered portal. (I am assuming that you are using a filter expression, not a filtered relationship). By copying the existing portal, it will have the same portal filter expression. Make it a one row portal. Put your summary field inside that one row portal and it will now display a total based on the records listed in the filtered portal.
This technique is "display only" there is no way to access that total for use in calculations, scripts, etc so if you need to do that, a different method for getting that total will be necessary.
Excellent, I have never seen that one before. Worked Great. Thanks