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 ? 
This is really the wrong list to be asking for help with basic DNS.
What you've described as what your customer wants is not possible.
Google: cname and other data
to learn why.
short answer: can't be done
You cannot have a cname and 'other' information for same entry. As a zone requires an SOA record, you cannot have a CNAME for the entire domain (theoretically a registrar could do it in .com, but afaik nobody does his).
Depending on customer's requirements, can put in a wildcard *.abcd.com in cname xyz.com
Or can load the same zone file for both abcd.com and xyz.com, so in both, mail<domain> goes to the same place
Confused
Do your customer want:
www.xyz.com pointing to the same IP as www.abcd.com
without having to manage xyz.com?