The most common and most likely answer to this crossword clue is the 3 plus 4 plus 4 plus 2 plus 4 plus 2 letter word NOT MAKE HEAD OR TAIL OF
I've seen this crossword clue in The Times
N
O
T
M
A
K
E
H
E
A
D
O
R
T
A
I
L
O
F
Buddy explains!
The straight part
We know that "Fail to understand" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fail to understand = not make head or tail of.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.