Sovereign measure (5)
I believe the answer is:
ruler
This clue is a double definition.
'sovereign' is the first definition.
(I've seen this before)
'measure' is the second definition.
(ruler is a kind of measure)
(Other definitions for ruler that I've seen before include "Straight edge; monarch" , "Person in authority" , "Strip used in drawing straight lines" , "One exercises dominion" , "Measuring stick" .)