Chief in rank (5)
I believe the answer is:
major
I believe this clue is a double definition.
'chief' is the first definition.
(I've seen this before)
'rank' is the second definition.
(I have seen 'Military rank' mean 'major' so perhaps 'rank' could also mean 'major')
'in' acts as a link.
(Other definitions for major that I've seen before include "grave" , "Leading - ex-Prime Minister" , "Important; old Animal Farm boar" , "Officer with crown" , "Of great importance" .)