It happened in my previous playthroughs that this particular claw was knocked off its pedestal either because of some area-effect spell (I think there's some such trap nearby) or because my characters used spells or shouts that knocked it down. IIRC, in that area you aren't just attacked by Draugr but also by a spell-hitting soul gem so a lot of combat happens there.
Unless you find it lying somewhere on the ground, you can spawn it with the console or use the console to teleport yourself to the claw and you'll find out its location.
The console command to add the glass claw to your inventory is player.additem 0007C260 1