Order ministry to occupy key officer (9)
I believe the answer is:
commodore
'officer' is the definition.
(commodore is a kind of officer)
'order ministry to occupy key' is the wordplay.
'order' becomes 'om' (Order of Merit).
'ministry' becomes 'mod' (Ministry of Defence).
'to occupy' means one lot of letters goes inside another (I've seen this in other clues).
'key' becomes 'core' (both can mean majorly important).
'om'+'mod'='ommod'
'ommod' put into 'core' is 'COMMODORE'.
(Other definitions for commodore that I've seen before include "senior officer" , "Naval officer" , "Naval rank junior to rear-admiral" , "President of a yacht club" , "officer, surely?" .)