Crossword Genius

Something used for measuring sovereign (5)

Ross

I believe the answer is:

ruler

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'sovereign' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for ruler that I've seen before include "liner?" , "Chief" , "King or, president, say" , "king perhaps" , "Monarch - straightedge" .)

I've seen this clue in The Daily Express.
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