The most common and most likely answer to this crossword clue is the 5 plus 3 plus 2 letter word DON’T ASK ME
I've seen this crossword clue in The Guardian
D
O
N
â
€
™
T
A
S
K
M
E
More clues leading to the result DON’T ASK ME
Buddy explains!

The straight part
We know that "I can’t be sure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i can’t be sure = don’t ask me.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.