The most common and most likely answer to this crossword clue is the 9 plus 6 letter word BENCHMARK FIXING
I've seen this crossword clue in Financial Times
B
E
N
C
H
M
A
R
K
F
I
X
I
N
G
Buddy explains!
The straight part
We know that "its credibility is shot on the 1s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case its credibility is shot on the 1s = benchmark fixing.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.