Get into alien territory (5)
I believe the answer is:
enter
'get' is the definition.
(thesaurus)
'into alien territory' is the wordplay.
'into' indicates the answer is hidden within the clue.
'ENTER' is hidden within 'alien territory'.
(Other definitions for enter that I've seen before include "Go in and put in the ledger" , "Set foot in" , "Join (for the first time)" , "Inscribe" , "Get in" .)