The most common and most likely answer to this crossword clue is the 9 letter word SEVENTEEN
I've seen this crossword clue in The Guardian
S
E
V
E
N
T
E
E
N
More clues leading to the result SEVENTEEN
- Number in competition observed going round
- A number spotted, happening to be paying visit
- A number observed fencing incident
- Prime competition watched outside
- Noticed something happening internally at this age
- A number noticed, happening to enter
- Prime case of extortion following incident in Home Counties
- Figure gathering spotted coats
- Number watched gripping episode
- One short of a majority?
Buddy explains!

The straight part
We know that "A number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a number = seventeen.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 SEVENTEEN mean?
SEVENTEEN numeral- The cardinal number occurring after sixteen and before eighteen, represented in Roman numerals as XVII and in Arabic numerals as 17.