Something used for measuring monarch (5)
I believe the answer is:
ruler
'monarch' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Queen maybe" , "Queen, say" , "Person commanding" , "It can be used to strike" , "one responsible for a line?" .)