Would need to see the photo to give the best advice... how big of a spot, where on the face, what is the angle of the face, etc.
Could try the patch tool to pick another spot of skin that is similarly toned to patch it in. Or if it's a straight-on shot, you could copy the good side of the face and flip it to mirror it on the other side (would work for just a spot on the face, but not the entire half, as our faces are not symmetrical).