The most common and most likely answer to this crossword clue is the 4 plus 2 plus 5 letter word I'LL BE BOUND
I
'
L
L
B
E
B
O
U
N
D
Buddy explains!
The straight part
We know that "I'm sure of it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i'm sure of it = i'll be bound.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.