Measure sovereign (5)
I believe the answer is:
ruler
I believe this is a double definition.
'measure' is the first definition.
(I know that ruler is a type of measure)
'sovereign' is the second definition.
(I've seen this before)
(Other definitions for ruler that I've seen before include "King or, president, say" , "Person in charge of country" , "One in power" , "One for drawing lines" , "Strip with straight edge to measure" .)