The most common and most likely answer to this crossword clue is the 7 letter word CERTAIN
I've seen this crossword clue in The Guardian
C
E
R
T
A
I
N
More clues leading to the result CERTAIN
- Iran etc. in turmoil for sure
- Guaranteed supply in crate
- Nuclear researchers keep time with artificial intelligence, for sure
- Convinced company’s boss ran it badly
- Could be dead start to afternoon at home for sure
- A murderer smothering monarch taking time to make sure
- In Paris, this accident in particular?
- I recant after conversion, for sure
- Certificate of film suitable for adults in particular
- I can be found in trance — in altered state, for sure
Buddy explains!

The straight part
We know that "sure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sure = certain.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.
What does CERTAIN mean?
CERTAIN noun- (with "the") Something certain.
- Sure, positive, not doubting.
- Determined; resolved.
- Not to be doubted or denied; established as a fact.
- Actually existing; sure to happen; inevitable.
- Unfailing; infallible.
- I have often wished , that I knew so certain a remedy in any other disease
- Fixed or stated; regular; determinate.
- Known but not specifically named; indeterminate; indefinite; one or some; sometimes used independently as a noun, and meaning certain persons; see also "one".
- (with of) Unnamed or undescribed members (of).