How to address a king, or a note to teacher (4)
I believe the answer is:
sire
'how to address a king' is the definition.
(I've seen this in another clue)
'a note to teacher' is the wordplay.
'a note' becomes 'e' (musical note).
'to' means one lot of letters go next to another (I've seen this in other clues).
'teacher' becomes 'sir' (way of addressing a male teacher).
'e' after 'sir' is 'SIRE'.
'or' acts as a link.
(Other definitions for sire that I've seen before include "Rise for male parent (4)." , "Old form of address to king - rise to it" , "Male parent of an animal" , "Ancestor" , "Male parent of a horse, for example" .)