The most common and most likely answer to this crossword clue is the 5 plus 8 letter word BASSO PROFUNDO
I've seen this crossword clue in Financial Times
B
A
S
S
O
P
R
O
F
U
N
D
O
More clues leading to the result BASSO PROFUNDO
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 = basso profundo.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.