The most common and most likely answer to this crossword clue is the 3 plus 3 letter word LET RIP
I've seen this crossword clue in Independent
L
E
T
R
I
P
More clues leading to the result LET RIP
- Triple jumping — go for it!
- Lash out, taking extremely lucrative journey
- On tour, the French abandon restraint
- Go wild seeing permit with tear
- Go off on one extremely large excursion
- Go to town three times, the last two prematurely
- Some risible tripartism can make you shout madly
- The French on holiday don’t hold back!
- Launch tirade: “Left rotten tripe!”
- The Parisian visit? Don’t hold back!
Buddy explains!
The straight part
We know that "sing heart out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sing heart out = let rip.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.