Common soldier to see detective (7,3)
I believe the answer is:
private eye
'detective' is the definition.
(thesaurus)
'common soldier to see' is the wordplay.
'common soldier' becomes 'private' ('private' can be a synonym of 'common soldier').
'to see' becomes 'eye' (I've seen this before).
'private'+'eye'='PRIVATE EYE'
(Other definitions for private eye that I've seen before include "investigator" , "supplement to work of police?" , "Periodical" , "Investigative satirical magazine" , "Satirical magazine - undercover detective" .)