The most common and most likely answer to this crossword clue is the 7 letter word PASSERS
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More clues leading to the result PASSERS
Buddy explains!

The straight part
We know that "potential witnesses" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case potential witnesses = passers.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 PASSERS mean?
PASSERS noun- One who succeeds in passing a test, etc.
- Someone who passes, someone who makes a pass.
- A passed pawn.
- One who passes; a passer-by.
- One who is able to "pass", or be accepted as a member of a race, sex or other group to which society would not otherwise regard them as belonging.