The most common and most likely answer to this crossword clue is the 2 plus 1 plus 4 letter word IN A WORD
I've seen this crossword clue in Independent
I
N
A
W
O
R
D
Buddy explains!
The straight part
We know that "Where a number of characters may be found/briefly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where a number of characters may be found/briefly = in a word.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.