Key officer (5)
I believe the answer is:
major
I believe this clue is a double definition.
'key' is the first definition.
(thesaurus)
'officer' is the second definition.
(type of military officer)
(Other definitions for major that I've seen before include "See 2" , "Officer - of greater importance" , "Military rank" , "Of considerable importance" , "Of great importance" .)