Go into forbidden territory (5)
I believe the answer is:
enter
'go' is the definition.
(thesaurus)
'into forbidden territory' is the wordplay.
'into' says the answer is hidden in the clue.
'ENTER' can be found hidden inside 'forbidden territory'.
(Other definitions for enter that I've seen before include "cross the threshold" , "Come or go in" , "Infiltrate" , "A key on the right" , "Gain access to" .)