Secret serviceman (7)
I believe the answer is:
private
I believe this clue is a double definition.
'secret' is the first definition.
(I know that secret can be written as private)
'serviceman' is the second definition.
(private is a kind of serviceman)
(Other definitions for private that I've seen before include "Personal or secret" , "Lowly soldier" , "(Of a place) secluded" , "Secret, not public" , "Lowest rank in the army" .)