Gain admission to some forbidden territory (5)
I believe the answer is:
enter
'gain admission' is the definition.
(I've seen this in another clue)
'some forbidden territory' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ENTER' is hidden in the letters of 'forbidden territory'.
'to' is the link.
(Other definitions for enter that I've seen before include "Treen (anag.)" , "Gain admission" , "Register (as a competitor)" , "Go in - set down" , "Make a record of" .)