The most common and most likely answer to this crossword clue is the 6 plus 7 plus 5 letter word ABSENT WITHOUT LEAVE
I've seen this crossword clue in The Times
A
B
S
E
N
T
W
I
T
H
O
U
T
L
E
A
V
E
Buddy explains!
The straight part
We know that "unexpectedly missing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unexpectedly missing = absent without leave.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.