Soldier’s secret? (7)
I believe the answer is:
private
I believe this is a double definition.
'soldier's' is the first definition.
(private is a type of soldier)
'secret?' is the second definition.
(I know that private can be written as private)
(Other definitions for private that I've seen before include "Arm y rank" , "See 16" , "Not to be shared or revealed" , "Secret sort of soldier" , "Rank, concealed" .)