Hi ,
To configure the clock rate for the hardware connections on serial interfaces such as network interface modules (NIMs) and interface processors to an acceptable bit rate, use the clock rate command in interface configuration mode. To remove the clock rate if you change the interface from a DCE to a DTE device, use the no form of this command. Using the no form of this command on a DCE interface sets the clock rate to the hardware-dependent default value.
Below link gives info about how to set clock rate for ATM interfaces and what is the need to setup clock rate..
http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/inter_r/int_a1g.htm#wp1142413
Hope it helps you.
Thanks,
satish