The most common and most likely answer to this crossword clue is the 4 plus 2 letter word CROP UP
I've seen this crossword clue in Independent
C
R
O
P
U
P
More clues leading to the result CROP UP
Buddy explains!

The straight part
We know that "to make unexpected appearance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to make unexpected appearance = crop up.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.