What does clear mean and at what age does it happen.?
Clear means "to change color". Your silver girl was born black. Over the course of time, she changed to silver. Given the light color of your girl, I would guess that at 8 weeks that her face, when shaved, was a very light color, possibly even platinum.
As she grew older, you probably saw her color clear from back to front and bottom to top. If you kept her cut short, this may not have been obvious to you. In a show dog the clearing can be quite dramatic because the bracelets will be silver, but the ears and top-knot will still be pewter (often with some unpleasant brown sun bleaching)
Forgive me for saying this, but you sound like you are getting ready to breed. As a breeder, your puppy buyers will expect you to be knowledgeable about the breed. They will expect you to serve as a mentor who can answer all their questions. Before you breed, I strongly urge you to become much more educated about Poodles.
Color is just the tip of the iceberg (yes, breeding silver to silver is the best way to get silver again). Health and testing are vitally important (did you know that VWBs is more prevalent in silvers?) What about training? Grooming? Have you considered joining a local Poodle Club?