The most common and most likely answer to this crossword clue is the 10 letter word TRESPASSED
I've seen this crossword clue in The Guardian
T
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Buddy explains!
The straight part
We know that "entered forbidden territory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case entered forbidden territory = trespassed.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRESPASSED mean?
TRESPASSED verb- To commit an offence; to sin.
- To offend against, to wrong (someone).
- To go too far; to put someone to inconvenience by demand or importunity; to intrude.
- To enter someone else's property illegally.
- To pass beyond a limit or boundary; hence, to depart; to go.
- To decree that a person shall be arrested for trespassing if he or she returns to someone else's land.