The most common and most likely answer to this crossword clue is the 5 letter word QUEER
I've seen this crossword clue in The Daily Telegraph
Q
U
E
E
R
More clues leading to the result QUEER
Buddy explains!
The straight part
We know that "Singular/ruin" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case singular/ruin = queer.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 QUEER mean?
QUEER noun- (sometimes derogatory) A person who is or appears homosexual, or who has homosexual qualities.
- (sometimes derogatory) A person of any non-heterosexual sexuality or sexual identity.
- (sometimes derogatory) A person of any genderqueer identity.
- (definite, with "the") Counterfeit money.
- To render an endeavor or agreement ineffective or null.
- To puzzle.
- To ridicule; to banter; to rally.
- To spoil the effect or success of, as by ridicule; to throw a wet blanket on; to spoil.
- To reevaluate or reinterpret (a work) with an eye to sexual orientation and/or to gender, as by applying queer theory.
- Weird, odd or different; whimsical.
- Slightly unwell (mainly in "to feel queer").
- Drunk.
- (sometimes derogatory) Homosexual.
- (sometimes derogatory) Not heterosexual, or not cisgender: homosexual, bisexual, asexual, transgender, etc.
- (broadly) Pertaining to sexual or gender behaviour or identity which does not conform to conventional heterosexual or cisgender norms, assumptions etc.
- Queerly.