Form of address on a note to teacher (4)
I believe the answer is:
sire
'form of address' is the definition.
'sire' can be an answer for 'address' (I have seen 'old address' mean 'sire' so perhaps 'address' could also mean 'sire'). I'm not sure about the 'form of' bit.
'note to teacher' is the wordplay.
'note' becomes 'e' (musical note).
'to' says to put letters next to each other (I've seen this in other clues).
'teacher' becomes 'sir' (way of addressing a male teacher).
'e' put after 'sir' is 'SIRE'.
'on a' acts as a link.
Can you help me to learn more?
(Other definitions for sire that I've seen before include "Old form of address to king - rise to it" , "Make children" , "Male parent of an animal" , "Male parent of a horse, for example" , "Ancestor" .)