The most common and most likely answer to this crossword clue is the 4 letter word ALTO
I've seen this crossword clue in The Times
A
L
T
O
More clues leading to the result ALTO
- A voice heard from chemical toilet
- How to introduce agent's least talented opera singer?
- Singer displaying some ethereal tones
- Singer, off-key a lot
- One singing in reverential tones
- Singer taking part in abysmal Tosca
- Singer — a lieutenant, old
- Singer frequently getting half confused
- Key Largo's ultimate voice
- Modifying key ought to get fairly high vocal range
Buddy explains!

The straight part
We know that "Singer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case singer = alto.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 ALTO mean?
ALTO noun- A musical part or section higher than tenor and lower than soprano, formerly the part that performed a countermelody above the tenor or main melody.
- A person or musical instrument that performs the alto part.
- An alto saxophone