Agent looks up number, rings immediately - artist volunteers and he's not wanted (7,3,5)
I believe the answer is:
persona non grata
'agent looks up number rings immediately artist volunteers and he's not wanted' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'number rings immediately artist volunteers and he's not wanted' is the wordplay.
I cannot quite understand how this works, but
'number' could be 'g' (G is an example) and 'g' is located in the answer.
'immediately' could be 'anon' (anon can mean soon or promptly) and 'anon' is present in the answer.
'artist' could be 'ra' (Royal Academician, member of Royal Academy of Arts) and 'ra' is found in the answer.
'volunteers' could be 'ta' (abbreviation for Territorial Army) and 'ta' is found in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
The remaining letters 'pers' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for persona non grata that I've seen before include "One banned" , "Undesirable" .)