The most common and most likely answer to this crossword clue is the 12 letter word PROPHYLACTIC
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Buddy explains!

The straight part
We know that "Deterrent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deterrent = prophylactic.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 PROPHYLACTIC mean?
PROPHYLACTIC noun- A medicine which preserves or defends against disease; a preventive.
- Any device or mechanism intended to prevent harmful consequences.
- Serving to prevent or protect against an undesired effect, especially disease.