Undisclosed military rank (7)
I believe the answer is:
private
This clue is a double definition.
'undisclosed' is the first definition.
(thesaurus)
'military rank' is the second definition.
(I know this)
(Other definitions for private that I've seen before include "Clandestine" , "See 16" , "Lowest ranking soldier" , "Not openly shared" , "Army three Dn" .)