The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 letter word SHOW NO PITY
I've seen this crossword clue in The Guardian
S
H
O
W
N
O
P
I
T
Y
Buddy explains!
The straight part
We know that "as instructed in the Old Testament" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as instructed in the old testament = show no pity.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.