Some territorials, travelling westward, retreat (4)
I believe the answer is:
lair
'retreat' is the definition.
(both can mean a private hideout)
'some territorials travelling westward' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'westward' says the letters should be written in reverse.
'rial' is hidden within 'territorials travelling'.
'rial' back-to-front is 'LAIR'.
(Other definitions for lair that I've seen before include "Private place of seclusion" , "Earth" , "Wild animals' habitation" , "Den of a wild animal" , "Hideaway, den" .)