The most common and most likely answer to this crossword clue is the 8 letter word RUTHLESS
I've seen this crossword clue in The Times
R
U
T
H
L
E
S
S
More clues leading to the result RUTHLESS
- Harsh description of defective Old Testament?
- Lacking mercy, like an incomplete Bible?
- Unforgiving if Bible is so defective?
- Savage not to be trusted after a scalping?
- The slurs being broadcast are really brutal
- Harsh regulations restricting time at hospital allotted to head of surgery
- Callous orders restricting most of the citizens, ultimately
- Criminal hustlers without mercy
- Hard-hearted? Girl not so much!
- Girl not so unmerciful
Buddy explains!

The straight part
We know that "Lacking mercy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking mercy = ruthless.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 RUTHLESS mean?
RUTHLESS adjective- Without pity or compassion; cruel, pitiless.