A straight-edge king (5)
I believe the answer is:
ruler
This clue is a double definition.
'a straight-edge' is the first definition.
(I've seen this before)
'king' is the second definition.
(I've seen this before)
(Other definitions for ruler that I've seen before include "Queen, say" , "Drawing stick or monarch" , "Monarch - straightedge" , "One who commands - often seen in the classroom" , "Potentate" .)