The most common and most likely answer to this crossword clue is the 10 letter word JAWBREAKER
I've seen this crossword clue in The Guardian
J
A
W
B
R
E
A
K
E
R
More clues leading to the result JAWBREAKER
Buddy explains!
The straight part
We know that "It’s hard to say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s hard to say = jawbreaker.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 JAWBREAKER mean?
JAWBREAKER noun- A large, extremely hard, boiled candy, typically spherical.
- A long, hard-to-pronounce word.