The most common and most likely answer to this crossword clue is the 2 plus 1 plus 4 plus 4 letter word BY A LONG SHOT
I've seen this crossword clue in The Daily Telegraph
B
Y
A
L
O
N
G
S
H
O
T
Buddy explains!
The straight part
We know that "That’s for sure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s for sure = by a long shot.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.