Native of cold territory, one descending, having crossed mountain to the west (9)
I believe the answer is:
laplander
'native of cold territory one' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'descending having crossed mountain to the west' is the wordplay.
'descending' becomes 'lander' (I am not sure about this - if you are sure you should believe this answer much more).
'having crossed' is an insertion indicator.
'mountain' becomes 'alp' (the Alps are a mountain range).
'to the west' says the letters should be written backwards (letters written right to left or 'east to west').
'alp' back-to-front is 'pla'.
'lander' enclosing 'pla' is 'LAPLANDER'.
Can you help me to learn more?
(Other definitions for laplander that I've seen before include "Santa's compatriot?" , "What falls easily to one" , "Northerner's" , "Northerner?" , "one in the far north" .)