The most common and most likely answer to this crossword clue is the 13 letter word UNACCOUNTABLE
I've seen this crossword clue in The Daily Telegraph
U
N
A
C
C
O
U
N
T
A
B
L
E
More clues leading to the result UNACCOUNTABLE
Buddy explains!
The straight part
We know that "that cannot be explained" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that cannot be explained = unaccountable.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 UNACCOUNTABLE mean?
UNACCOUNTABLE adjective- Inexplicable; unable to account for, or explain.
- Not responsible; free from accountability or control.