Monarchs’ measuring devices (6)
I believe the answer is:
rulers
'monarchs'' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rulers that I've seen before include "Drawing sticks or governors" , "Drawing sticks or monarchs" , "Strips with straight edge to measure" , "Kings, queens, presidents etc" , "they must not be crooked" .)