Officer of the crown (5)
I believe the answer is:
major
'officer' is the definition.
(type of military officer)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for major that I've seen before include "grave" , "5,27 [PRIME MINISTER]" , "Important, significant" , "Chief - officer" , "Of great importance" .)