The most common and most likely answer to this crossword clue is the 1 plus 4 plus 3 letter word I TELL YOU
I've seen this crossword clue in The Sunday Telegraph
I
T
E
L
L
Y
O
U
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! = i tell you.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.