The most common and most likely answer to this crossword clue is the 5 plus 4 letter word FALSE DAWN
I've seen this crossword clue in Financial Times
F
A
L
S
E
D
A
W
N
More clues leading to the result FALSE DAWN
- Sporting body with change of law ends time of illusory promise
- Lands a few blows in a deceptively promising period
- Crawl over dales, distraught when promise is unfulfilled
- It will disappoint very little wife in large chair
- Wand revealing transient light?
- Servile flattery about dubious deals in promising start that flops
- Wand perhaps giving deceptive appearance
- Deceptive image misleading a few lands
- Treacherous French promise of fresh start coming to nothing
- Sandy going round London university in a day, giving deceptive impression
Buddy explains!
The straight part
We know that "when promise is unfulfilled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when promise is unfulfilled = false dawn.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.