King at Christmas starts to get upset during service, missing son (5)
I believe the answer is:
magus
'son' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for magus that I've seen before include "Magician, wise man" , "Sorcerer" , "Wizard" , "astrologer" , "Balthazar, Gaspar or Melchior individually" .)