The most common and most likely answer to this crossword clue is the 8 letter word OMITTING
I've seen this crossword clue in The Daily Telegraph
O
M
I
T
T
I
N
G
More clues leading to the result OMITTING
Buddy explains!

The straight part
We know that "Promising" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case promising = omitting.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 OMITTING mean?
OMITTING verb- To leave out or exclude.
- To fail to perform.
- To neglect or take no notice of.
- An act of omission.