The most common and most likely answer to this crossword clue is the 8 letter word EASEMENT
I've seen this crossword clue in The Guardian
E
A
S
E
M
E
N
T
More clues leading to the result EASEMENT
- Release men thus possessing a legal right
- European window destroys Conservative peace
- A softly-softly falling out, conciliation brings peace
- Soldiers in each group will get legal right
- Legal right allowing oriental fellows to occupy their quarter
- Relief to chew over source of life
- In leisure time, people right to cross land
- English people in Orient getting right over neighbouring land
- Me sporting tailored sateen? Kind of right
- Land right in the middle of green with meanest shot
Buddy explains!

The straight part
We know that "right to use what is not theirs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case right to use what is not theirs = easement.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 EASEMENT mean?
EASEMENT noun- The legal right to use another person's real property (real estate), generally in order to cross a part of the property or to gain access to something on the property (right of way).
- An element such as a baseboard, handrail, etc., that is curved instead of abruptly changing direction.
- Easing, relief.
- The act of relieving oneself: defecating or urinating.
- (model railroading) Transition spiral curve track between a straight or tangent track and a circular curved track of a certain radius or selected radius.
- Assistance.
- Support.
- Gratification.