The most common and most likely answer to this crossword clue is the 7 letter word PRICKLY
I've seen this crossword clue in The Guardian
P
R
I
C
K
L
Y
Buddy explains!

The straight part
We know that "Difficult" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case difficult = prickly.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 PRICKLY mean?
PRICKLY noun- Something that gives a pricking sensation; a sharp object.
- Covered with sharp points.
- Easily irritated.
- Difficult; complicated; (figuratively) hairy or thorny.
- In a prickly manner.