The most common and most likely answer to this crossword clue is the 15 letter word UNPRONOUNCEABLE
I've seen this crossword clue in The Sunday Telegraph
U
N
P
R
O
N
O
U
N
C
E
A
B
L
E
Buddy explains!
The straight part
We know that "It’s impossible to say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s impossible to say = unpronounceable.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 UNPRONOUNCEABLE mean?
UNPRONOUNCEABLE noun- Something difficult or impossible to pronounce.
- Impossible or difficult to pronounce or articulate.