Go into some forgotten territory (5)
I believe the answer is:
enter
'go into' is the definition.
(enter can mean to go into a place)
'some forgotten territory' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ENTER' can be found hidden inside 'forgotten territory'.
(Other definitions for enter that I've seen before include "Infiltrate" , "Go in - set down" , "Fill in" , "Break in" , "Go in and put in the ledger" .)