Clawhammer is/was the codename for the first 64-bit AMD processor. Clawhammer will (was to be?) be aimed at the desktop and low-end server market, while the more powerful sledgehammer will target the high-end SMP server market. The first clawhammers were expected to hit the market in the first quarter of 2002, however, nothing resembling clawhammer has shipped yet (as of March, 2003). As far as I can tell, the "clawhammer" and "sledgehammer" codenames have been dropped by AMD, as no articles published since early 2002 mention them.
See also: Sledgehammer, IA-64, Itanium