The most common and most likely answer to this crossword clue is the 14 letter word INDETERMINABLE
I've seen this crossword clue in Financial Times
I
N
D
E
T
E
R
M
I
N
A
B
L
E
Buddy explains!
The straight part
We know that "cannot be ascertained" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cannot be ascertained = indeterminable.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 INDETERMINABLE mean?
INDETERMINABLE noun- An indeterminable thing or quantity.
- That is incapable of being measured.
- That is incapable of being ascertained.