The most common and most likely answer to this crossword clue is the 6 letter word FIZZLE
I've seen this crossword clue in The Guardian
F
I
Z
Z
L
E
More clues leading to the result FIZZLE
Buddy explains!
The straight part
We know that "die, having promised much" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case die, having promised much = fizzle.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 FIZZLE mean?
FIZZLE noun- A spluttering or hissing sound.
- Failure of a nuclear bomb to meet its expected yield during testing.
- An abortive effort; a flop or dud.
- A state of agitation or worry.
- To sputter or hiss.
- To decay or die off to nothing; to burn out; to end less successfully than previously hoped.