No petty officer! (5)
I believe the answer is:
major
'officer' is the definition.
(type of military officer)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for major that I've seen before include "Significant, main" , "Chief - officer" , "See 11 Across" , "Conservative PM" , "Important - ex-PM" .)