Student's notes on boy king (5)
I believe the answer is:
tutee
'student's' is the definition.
(someone being tutored)
'notes on boy king' is the wordplay.
'notes' becomes 'ee' (two E's - as in the musical note E).
'on' says to put letters next to each other.
'boy king' becomes 'tut' (I've seen this in another clue).
'ee' put after 'tut' is 'TUTEE'.
(Other definitions for tutee that I've seen before include "One learns from a personal teacher" , "Student, pupil" .)