[Brad Schimanski]

found this on a neighbouring site: works a charm and far easier than some of the options below.
https://www.damonledet.com/2008/06/after-effects-lens-flare-with-transparent-background/

Step one: Create a solid and add the lens Flare filter to it, get the lens flare just how you would like it and duplicate the solid.
Step two: Set the bottom solid to use the top solid as a track matte (Luma Matte)

That’s it transparent lens flare.

there was a bit more too it but thats all you need.

hope it helps

Brad