Commander in chief? (5)
I believe the answer is:
major
I believe this clue is a double definition.
'commander' is the first definition.
(I know that commander can be written as major)
'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 "See 2" , "Former PM" , "Important - ex-PM" , "Pre-eminent" , "Significant; rank" .)