Serving in them is a private job (5)
I believe the answer is:
ranks
'serving in them' is the definition.
I can't tell whether this definition defines the answer.
'a private job' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ranks that I've seen before include "Rows (of soldiers)" , "Positions of seniority" , "Classifies" , "Lines or rows of people or things" , "Grades, in armed forces say" .)