Go into some alien territories (5)
I believe the answer is:
enter
'go into' is the definition.
(enter can mean to go into a place)
'some alien territories' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ENTER' can be found hidden inside 'alien territories'.
(Other definitions for enter that I've seen before include "Take part in a contest" , "Come/go in" , "Come or go in" , "Go/come in" , "Register formally as a participant" .)