The most common and most likely answer to this crossword clue is the 12 letter word UNIMAGINABLE
I've seen this crossword clue in The Times
U
N
I
M
A
G
I
N
A
B
L
E
Buddy explains!
The straight part
We know that "Difficult to comprehend" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case difficult to comprehend = unimaginable.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 UNIMAGINABLE mean?
UNIMAGINABLE adjective- Unable to be imagined; inconceivable or mind-boggling; beyond belief