Measure of monarch? (5)
I believe the answer is:
ruler
I believe this clue is a double definition.
'measure' is the first definition.
(ruler is a kind of measure)
'monarch?' is the second definition.
(thesaurus)
'of' is the link.
(Other definitions for ruler that I've seen before include "establishing a line?" , "Potentate" , "one responsible for a line?" , "Monarch - drafting tool" , "Strip used in drawing straight lines" .)