CNAME issue

Hi All, One of our customer having the following requirement. There is a domain abcd.com ( zone file created , A records are pointed ). He has another domain xyz.com. He want us to create a separate zone file for " xyz.com " & abcd.com should be the CNAME of it. ( No "A" records mentioned ) I'm bit confuse about adding this in zone file. Can any one help ? :( -- ~~( ŊëŌ )~~