The most common and most likely answer to this crossword clue is the 8 letter word PUNCTUAL
P
U
N
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More clues leading to the result PUNCTUAL
Buddy explains!

The straight part
We know that "prompt" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prompt = punctual.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 PUNCTUAL mean?
PUNCTUAL adjective- Prompt; on time.
- Existing as a point or series of points
- Expressing momentary action that has no duration
- (Euro-English) Periodic; occasional.
- Observing trivial points; punctilious.