Apply the Circle Wipe Effect, and keep in mind that you will have to experiment somewhat to figure out just how many frames you need total for the effect. Create keyframes (enter Effect Mode and click the “Add Keyframe” button underneath the right-side, aka “record” side of the Composer monitor) in the middle of the effect to hold the wipe on your subject’s face. Adjust the keyframes as necessary using the controls in the Effect Editor. I don’t know if it will let you adjust the position of the circle wipe (I’ve never tried it and I’m not in front of my Avid), so there’s a chance the wipe will only be in the exact center of the screen. If that’s the case, and your subject doesn’t fall within that area, you might not be able to use this effect to achieve what it is you’re hoping for…
…however, if you’re on Avid XPro or above, go into the Effect Palette, click on the “Illusion FX” category, and select the Iris effect…which will give you EXACTLY what you want. Keyframe it the same way as above, and if I remember correctly, it will allow you to place the position of the circle you want, allowing you to keyframe it exactly where you want it and for the duration you want it.
Consult the manual on creating/editing effects if you need more help. You can access the online version from the Help Menu at the top-left of your screen.
Hope that helps!