The most common and most likely answer to this crossword clue is the 11 letter word INSCRUTABLE
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result INSCRUTABLE
Buddy explains!

The straight part
We know that "difficult to understand" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case difficult to understand = inscrutable.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 INSCRUTABLE mean?
INSCRUTABLE noun- One who or that which is inscrutable; a person, etc. that cannot be comprehended.
- Difficult or impossible to comprehend, fathom or interpret.