Order a chap to be king of the fairies! (6)
I believe the answer is:
oberon
'king of the fairies' is the definition.
(I've seen this before)
'order a chap to be' is the wordplay.
'order' becomes 'o' (abbreviation e.g. in British honours OM, OBE etc.).
'a chap' becomes 'ron' (short for Ronald).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'ron' after 'be' is 'beron'.
'o'+'beron'='OBERON'
(Other definitions for oberon that I've seen before include "Play King" , "charming king" , "Shakespeare's fairy king" , "Fairy king (AMND)" , "Satellite" .)