They produce straight lines of monarchs (6)
I believe the answer is:
rulers
'monarchs' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rulers that I've seen before include "they must not be crooked" , "Measuring sticks" , "Strips with straight edge to measure" , "Drawing sticks or monarchs" , "Sticks marked off in units" .)