The most common and most likely answer to this crossword clue is the 5 plus 9 letter word ABOVE SUSPICION
I've seen this crossword clue in The Guardian
A
B
O
V
E
S
U
S
P
I
C
I
O
N
Buddy explains!
The straight part
We know that "beyond reproach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beyond reproach = above suspicion.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.