Apex domains cannot have CNAME records.
Question and response
Question
dig cl.cam.ac.uk. NS
Answer
cl.cam.ac.uk. 3 NS ns2.ic.ac.uk.
cl.cam.ac.uk. 3 NS auth0.dns.cam.ac.uk.
cl.cam.ac.uk. 3 NS dns0.eng.cam.ac.uk.
cl.cam.ac.uk. 3 NS ns1.mythic-beasts.com.
cl.cam.ac.uk. 3 NS dns0.cl.cam.ac.uk.
cl.cam.ac.uk. 3 NS ns3.mythic-beasts.com.
cl.cam.ac.uk. 3 NS dns1.cl.cam.ac.uk.
Authority
The response has no authority records.
Additional
ns1.mythic-beasts.com. 3 A 45.33.127.156
ns2.ic.ac.uk. 3 A 155.198.142.82
ns3.mythic-beasts.com. 3 A 185.24.221.32
dns0.cl.cam.ac.uk. 3 A 128.232.0.19
dns0.eng.cam.ac.uk. 3 A 129.169.8.8
auth0.dns.cam.ac.uk. 3 A 131.111.8.37
ns1.mythic-beasts.com. 3 AAAA 2600:3c00:e000:19:0:0:0:1
ns2.ic.ac.uk. 3 AAAA 2a0c:5bc0:4:1:0:0:0:82
ns3.mythic-beasts.com. 3 AAAA 2a02:2770:11:0:21a:4aff:febe:759b
dns0.cl.cam.ac.uk. 3 AAAA 2a05:b400:110:0:0:0:d:a0
auth0.dns.cam.ac.uk. 3 AAAA 2001:630:212:8:0:0:d:a0
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0