Officer in chief (5)
I believe the answer is:
major
This clue is a double definition.
'officer' is the first definition.
(major is a kind of officer)
'chief' is the second definition.
(I've seen this before)
'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" .)