The most common and most likely answer to this crossword clue is the 5 letter word CRUEL
I've seen this crossword clue in The Daily Telegraph
C
R
U
E
L
More clues leading to the result CRUEL
- Regret cutting first half of clue, causing distress
- . Nasty about French street – lying primarily
- Regret punching Conservative and Liberal, causing pain
- Ruthless Conservative line taken without regret
- Hard clue set to trap solver ultimately
- Captain to regret loss that’s heartbreaking
- Be sorry in class as opposed to being mean
- Cold-hearted captain’s rule goes astray
Buddy explains!

The straight part
We know that "Ruthless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ruthless = cruel.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 CRUEL mean?
CRUEL verb- To spoil or ruin (one's chance of success)
- To violently provoke (a child) in the belief that this will make them more assertive.
- Intentionally causing or reveling in pain and suffering; merciless, heartless.
- Harsh; severe.
- Cool; awesome; neat.
- To a great degree; terribly.
- Worsted yarn, slackly twisted, used for embroidery.