Old soldiers mostly out of it in a state (6)
I believe the answer is:
oregon
'a state' is the definition.
(Oregon is an example)
'old soldiers mostly out of it' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'soldiers' becomes 'RM' (Royal Marines).
'mostly' means to remove the last letter (most but not all of the word).
'out of it' becomes 'egon' (I can't explain this - if you can you should believe this answer much more).
'rm' with its last letter taken away is 'r'.
'o'+'r'+'egon'='OREGON'
'in' is the link.
Can you help me to learn more?
(Other definitions for oregon that I've seen before include "Part of the US" , "No gore seen in this American state" , "US region" , "Old trail led here" , "West coast US state" .)