Officer in chief (5)
I believe the answer is:
major
I believe this is a double definition.
'officer' is the first definition.
(type of military officer)
'chief' is the second definition.
(I've seen this in another clue)
'in' is the link.
(Other definitions for major that I've seen before include "Big - PM" , "Former PM" , "Not minor" , "Main course" , "Of great importance" .)