The most common and most likely answer to this crossword clue is the 5 plus 10 letter word CLOSE ENCOUNTERS
I've seen this crossword clue in Independent
C
L
O
S
E
E
N
C
O
U
N
T
E
R
S
More clues leading to the result CLOSE ENCOUNTERS
Buddy explains!
The straight part
We know that "in which aliens may appear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in which aliens may appear = close encounters.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.