Go in some part of alien territory (5)
I believe the answer is:
enter
'go in' is the definition.
(as in entering a building)
'some part of alien territory' is the wordplay.
'some part of' says the answer is hidden in the clue.
'ENTER' can be found hidden inside 'alien territory'.
(Other definitions for enter that I've seen before include "A key on the right" , "Move into" , "Make a record of" , "Come in - penetrate - record" , "Enrol" .)