Crossword Genius

First among ministers (5)

Ross

I believe the answer is:

prime

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'first' is the definition.
(similar in meaning)

I don't understand how the remainder of the clue works.

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(Other definitions for prime that I've seen before include "Fundamental - of highest quality" , "Original" , "(Of number) divisible only by itself and one" , "Maybe eleven" , "Main; make ready" .)

I've seen this clue in the Evening Standard.
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