The most common and most likely answer to this crossword clue is the 5 letter word FLUNK
F
L
U
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K
Buddy explains!

The straight part
We know that "failure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case failure = flunk.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 FLUNK mean?
FLUNK verb- Of a student, to fail a class; to not pass.
- Of a teacher, to deny a student a passing grade.
- To shirk (a task or duty).
- To back out through fear. (Commonly in the phrase 'flunk it', the 'it' referring to a specific task avoided; sometimes without specific reference describing a person's attitude to life in general.)