Okay, first of all, it's not difficult to come up with a metaphor for clean. What's difficult is to narrow it down to one. So here's my first one, and there will be others, as I can articulate them:

Clean is like an empty stage, accoustically live, that allows me the space, the time, and the opportunity to hear myself. The facilitator is somewhere in the audience in the darkness beyond the stage lights, listening, but staying out of the way of my experience of my own inner being. Somehow, in this place, I can hear and understand better than anywhere else the sounds and the silences.