The most common and most likely answer to this crossword clue is the 5 letter word VOUCH
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More clues leading to the result VOUCH
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 = vouch.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 VOUCH mean?
VOUCH noun- Warrant; attestation.
- To take responsibility for; to express confidence in; to witness; to obtest.
- To warrant; to maintain by affirmations
- To back; to support; to confirm.
- To call into court to warrant and defend, or to make good a warranty of title.
- To call; to summon.
- To bear witness; to give testimony or full attestation.
- To call as a witness.
- To assert; to aver; to declare.