Monarch drawing aid (5)
I believe the answer is:
ruler
I believe this is a double definition.
'monarch' is the first definition.
(I've seen this before)
'drawing aid' is the second definition.
(I've seen this before)
(Other definitions for ruler that I've seen before include "Measuring rod; governor" , "Strip with straight edge to measure" , "Monarch - drafting tool" , "person holding sway?" , "One measures" .)