Apex domains cannot have CNAME records.
Question and response
Question
dig clusters.ovh. NS
Answer
clusters.ovh. 5 NS ns105.ovh.net.
clusters.ovh. 5 NS dns105.ovh.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0