Officer on his own in pass (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'on his own in pass' is the wordplay.
'on his own' becomes 'lone' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'pass' becomes 'col' (col is a term for a mountain pass).
'lone' put inside 'col' is 'COLONEL'.
Can you help me to learn more?
(Other definitions for colonel that I've seen before include "Military rank" , "Comissioned military officer" , "'No cello for this officer, strangely (7)'" , "Commander" , "Army officer ranking below a brigadier" .)