The most common and most likely answer to this crossword clue is the 10 letter word VISITATION
I've seen this crossword clue in The Guardian
V
I
S
I
T
A
T
I
O
N
Buddy explains!

The straight part
We know that "appearance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case appearance = visitation.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 VISITATION mean?
VISITATION noun- The act of visiting, or an instance of being visited.
- An official visit to inspect or examine something.
- An encounter with supernatural beings such as ghosts or aliens.
- The right of a separated or divorced parent to visit a child; access.
- A punishment or blessing ordained by God.
- An unusual and extensive irruption of a species of animals into another region.