Crossword Genius

No petty officer! (5)

Ross

I believe the answer is:

major

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'officer' is the definition.
(type of military officer)

I don't understand the remainder of the clue.

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(Other definitions for major that I've seen before include "Significant, main" , "Chief - officer" , "See 11 Across" , "Conservative PM" , "Important - ex-PM" .)

I've seen this clue in the Evening Standard.
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