Officer in the Blues? (5)
I believe the answer is:
major
'officer' is the definition.
(major is a kind of officer)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for major that I've seen before include "Former PM" , "Important - ex-PM" , "Officer - of greater importance" , "Significant; rank" , "Of considerable importance" .)