Apex domains cannot have CNAME records.
Question and response
Question
dig clients.your-server.de. NS
Answer
clients.your-server.de. 2 NS ns.second-ns.com.
clients.your-server.de. 2 NS ns1.your-server.de.
Authority
The response has no authority records.
Additional
ns.second-ns.com. 1 A 213.239.204.242
ns1.your-server.de. 2 A 213.133.100.102
ns.second-ns.com. 1 AAAA 2a01:4f8:0:a101:0:0:b:1
ns1.your-server.de. 2 AAAA 2a01:4f8:0:1:0:0:5ddc:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0