The most common and most likely answer to this crossword clue is the 8 letter word STRANGER
I've seen this crossword clue in The Times
S
T
R
A
N
G
E
R
More clues leading to the result STRANGER
- Way to offend entertaining royal outsider?
- Unfamiliar visitor starts to suspect the park keeper
- Patrolman on way shows new face
- Visitor called cutting rest, unfortunately
- Throttling killer disposing of line is not so normal
- Keeper on second team originally unknown
- More unusual way taken by park-keeper
- Outsider arresting criminal? Not I
- Relatively unknown economist ran German banks
- Comparatively odd murderer drops pound
Buddy explains!

The straight part
We know that "unknown" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unknown = stranger.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 STRANGER mean?
STRANGER adjective- Not normal; odd, unusual, surprising, out of the ordinary.
- Unfamiliar, not yet part of one's experience.
- Having the quantum mechanical property of strangeness.
- Of an attractor: having a fractal structure.
- Belonging to another country; foreign.
- Reserved; distant in deportment.
- Backward; slow.
- Not familiar; unaccustomed; inexperienced.
- Not belonging to one.
- A person whom one does not know; a person who is neither a friend nor an acquaintance.
- An outsider or foreigner.
- One not admitted to communion or fellowship.
- A newcomer.
- One who has not been seen for a long time.
- One not belonging to the family or household; a guest; a visitor.
- One not privy or party to an act, contract, or title; a mere intruder or intermeddler; one who interferes without right.
- A superstitious premonition of the coming of a visitor by a bit of stalk in a cup of tea, the guttering of a candle, etc.
- To estrange; to alienate.