clip on the time line on track one and comp mode to hard light add levels fx and bump up Gamma to normal light or lower for night city feel
on track 2 place white media generator panel for a few frames where you want the flash to go off
this way its easy to set the flashes to a beat
you can also use elliptical white med gen instead of a solid panel, leave the black for a rounded flash
edit
you can also use the same method for colored light's flashing just change the color of the white media panel to the required clour
I usually snip the footage 6 frames to the right of the frame I want to simulate flash on.
the part you snipped I pull back 12 frames to the left and overlap the two by 12 frames. I then use the flash transition - soft for the 12 frames and it simulates a camera flash.
Although a camera flash is only 1 frame in reality, the 12 frames gives it more of a noticable flash.
That's what I do anyway....
If I want to pretend that the live footage now becomes a picture by camera flash that is what I do but I take a capture of that 6th frame in and over lap that instead of the rest of the footage.