Crossword Genius

First officer who looks after himself, primarily? (6,3)

Ross

I believe the answer is:

number one

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'first' is the definition.
(synonyms)

I don't understand the rest of the clue.

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(Other definitions for number one that I've seen before include "Cut" , "Informally and numerically, oneself" , "Top-of-the-charts hit" , "leader" , "top-selling record" .)

I've seen this clue in The Guardian.
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