Registered name is 'Earl Z'! (8)
I believe the answer is:
enrolled
'registered' is the definition.
(similar in meaning)
'name is earl z' is the wordplay.
I cannot really understand how this works, but
'name' could be 'ed' (short for Edward) and 'ed' is found within the answer.
'earl' could be 'e' and 'e' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for enrolled that I've seen before include "Became a member" , "Accepted into organisation" , "Registered,entered" , "Registered as a member" , "Entered in the register" .)