The most common and most likely answer to this crossword clue is the 6 plus 7 letter word TONGUE TWISTER
I've seen this crossword clue in The Daily Telegraph
T
O
N
G
U
E
T
W
I
S
T
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R
Buddy explains!
The straight part
We know that "What’s difficult to say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s difficult to say = tongue twister.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 TONGUE TWISTER mean?
TONGUE TWISTER noun- A phrase that is deliberately designed to be difficult to say correctly, usually because of varying combinations of similar phonemes.