Check to make sure the switch is not grounding to the weapon frame. I sounds like when the clamp is tightened, it makes contact through the anodizing and something is shorting out via the clamp / laser / switch.
I agree it's unlikely that the clamp is distorting anything.
How is the switch mounted? Or did you create your own trigger switch? It's possible that the laser barrel is connected to battery positive and the switch is in the negative leg (or vice versa). If the switch is connected to weapon ground, it will not work when everything is married up.
To troubleshoot this, you could try taping the laser barrel with some polyimide (kapton) or mylar tape, this stuff is very thin yet a good insulator. If this material is not readily available to you, use some thin insulating sheet stock between the clamp and laser barrel. If you can clamp it down with the insulator in place and it still works, it was a short and you need to take another look at how the switch is mounted / connected.
