To a soldier, denotes unearned income (7,5)
I believe the answer is:
private means
'unearned income' is the definition.
(I've seen this before)
'to a soldier denotes' is the wordplay.
'to a soldier' becomes 'private' (private is a type of soldier).
'denotes' becomes 'means' (I've seen this in another clue).
'private'+'means'='PRIVATE MEANS'
(Other definitions for private means that I've seen before include "source of income" , "Income other than salary" , "Unearned income" , "make money from investments" .)