The most common and most likely answer to this crossword clue is the 4 letter word LEST
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More clues leading to the result LEST
Buddy explains!

The straight part
We know that "in case" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in case = lest.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 LEST mean?
LEST conjunction- For fear that; that not; in order to prevent something from happening; in case.
- (after certain expressions denoting fear or apprehension) that (without the negative particle; introduces the reason for an emotion.)