Once the guard is press fit to its final position, I remove the guard, completly finish the front, and then put it back in place. If everything is nice and snug in the fit, the pressure of the fit will hold the guard in place.
Just prior to gluing on the handle, I coat the entire front of the guard with vaseline. If you fill up the hole in the handle material 2/3-3/4 with glue, when you insert the tang, hydraulic action will force glue into any tiny spaces between the guard and tang. IF any glue seeps through the guard/tang fit, because the vaseline is there, it has nothing to "grip" onto. Once cured, you can use a sharpened brass rod to just "pop" any glue off.
I usual encouarge people NOT to use "clamps" when it comes to a hidden tang, because more often then not, the clamping pressure isn't even, and once the glue cures, you will often times find gaps or odd fits due to the clamping pressure. The best method I've found is to ensure that the angle between the back of the guard, and the front of the handle material is as close to perfect as you can make it.......then, when you insert/glue the tang into the handle material set it with the handle material end on the bench, and let the weight of the blade & guard do the work. The most I will ever to for a "clamp" is to sometimes use two rubber bands......from the top and bottom of the guard, and over the end of the handle material.