Secret soldier of the lowest rank (7)
I believe the answer is:
private
This might be a double definition.
'secret' is the first definition.
(I know that secret can be written as private)
'soldier of the lowest rank' is the second definition.
'private' can be an answer for 'rank' (I've seen this before). I'm unsure of the 'soldier of the lowest' bit.
Can you help me to learn more?
(Other definitions for private that I've seen before include "Not open to scrutiny" , "Army rank; secluded" , "To most, unknown" , "Not to be shared or revealed" , "See 16" .)