The most common and most likely answer to this crossword clue is the 9 letter word VOUCHSAFE
V
O
U
C
H
S
A
F
E
More clues leading to the result VOUCHSAFE
Buddy explains!

The straight part
We know that "guarantee" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case guarantee = vouchsafe.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 VOUCHSAFE mean?
VOUCHSAFE verb- To graciously give, to condescendingly grant a right, benefit, outcome, etc.; to deign to acknowledge.
- To receive or accept in condescension.
- To disclose or divulge.