The most common and most likely answer to this crossword clue is the 6 plus 2 plus 7 letter word THERES NO TELLING
I've seen this crossword clue in The Daily Telegraph
T
H
E
R
E
S
N
O
T
E
L
L
I
N
G
Buddy explains!
The straight part
We know that "It’s impossible to predict" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s impossible to predict = theres no telling.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.