The most common and most likely answer to this crossword clue is the 11 letter word JAW-BREAKER
I've seen this crossword clue in Financial Times
J
A
W
-
B
R
E
A
K
E
R
More clues leading to the result JAW-BREAKER
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 = jaw-breaker.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.