Wrong thing to do in forbidden territory (5)

I believe the answer is:

enter

I'm a little stuck... Click here to teach me more about this clue!

'wrong thing to do' is the definition.
I can't judge whether this definition defines the answer.

'in forbidden territory' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ENTER' can be found hidden inside 'forbidden territory'.

Can you help me to learn more?

(Other definitions for enter that I've seen before include "Fruit drink" , "submit?" , "Record (in a log)" , "Pierce" , "Go/come in" .)

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