Apex domains cannot have CNAME records.
Question and response
Question
dig clusterdns.top. NS
Answer
clusterdns.top. 5 NS nick.ns.cloudflare.com.
clusterdns.top. 5 NS gwen.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0