The most common and most likely answer to this crossword clue is the 5 plus 9 letter word CLOSE ENCOUNTER
C
L
O
S
E
E
N
C
O
U
N
T
E
R
Buddy explains!
The straight part
We know that "UFO sighting?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ufo sighting? = close encounter.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.