Go into some forbidden territory (5)
I believe the answer is:
enter
'go into' is the definition.
(enter can mean to go into a place)
'some forbidden territory' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ENTER' is hidden within 'forbidden territory'.
(Other definitions for enter that I've seen before include "Gain access to" , "Infiltrate" , "put in for a race" , "Set foot in" , "Join (for the first time)" .)