The most common and most likely answer to this crossword clue is the 3 plus 5 plus 3 plus 4 letter word YOU NEVER CAN TELL
I've seen this crossword clue in The Times
Y
O
U
N
E
V
E
R
C
A
N
T
E
L
L
Buddy explains!
The straight part
We know that "I'm not sure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i'm not sure = you never can tell.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.