The most common and most likely answer to this crossword clue is the 5 plus 5 plus 4 letter word BREAK ONE’S WORD
I've seen this crossword clue in The Guardian
B
R
E
A
K
O
N
E
â
€
™
S
W
O
R
D
Buddy explains!
The straight part
We know that "fail test of reliability" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fail test of reliability = break one’s word.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.