Mountainous region, to the west, Mallory traversed partly (5)
I believe the answer is:
tyrol
'mountainous region' is the definition.
(I've seen this before)
'to the west mallory traversed partly' is the wordplay.
'to the west' says the letters should be written backwards (letters written right to left or \'east to west\').
'partly' indicates a hidden word.
'loryt' is hidden within 'mallory traversed'.
'loryt' in reverse letter order is 'TYROL'.
(Other definitions for tyrol that I've seen before include "Picturesque mountain area in Austria, popular with skiers" , "Alpine region of Austria and Italy" , "Austrian alpine region" , "ski region" , "hills here" .)