Monarch's measuring device (5)
I believe the answer is:
ruler
I believe this clue is a double definition.
'monarch's' is the first definition.
(I've seen this before)
'measuring device' is the second definition.
(I've seen this in another clue)
(Other definitions for ruler that I've seen before include "Leader - line-drawing aid" , "one going straight?" , "Queen, say" , "Maharaja, e.g." , "Thoroughly straight" .)