Teacher openly revered at first by boy king (5)
I believe the answer is:
tutor
'teacher' is the definition.
(I know that tutor is a type of teacher)
'openly revered at first by boy king' is the wordplay.
'at first' indicates taking the first letters.
'by' means one lot of letters go next to another.
'boy king' becomes 'Tut' (Tutankhamun).
The initial letters of 'openly revered' is 'or'.
'or' after 'tut' is 'TUTOR'.
(Other definitions for tutor that I've seen before include "Private or university teacher" , "Teach" , "Personal coach" , "School" , "Guardian" .)