The most common and most likely answer to this crossword clue is the 10 letter word AUSPICIOUS
I've seen this crossword clue in The Times
A
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More clues leading to the result AUSPICIOUS
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 = auspicious.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 AUSPICIOUS mean?
AUSPICIOUS adjective- Of good omen; indicating future success.
- Conducive to success.
- Marked by success; prosperous.