The most common and most likely answer to this crossword clue is the 10 letter word LOOKERS-ON
I've seen this crossword clue in The Guardian
L
O
O
K
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More clues leading to the result LOOKERS-ON
Buddy explains!
The straight part
We know that "Witnesses" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case witnesses = lookers-on.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 LOOKERS-ON mean?
LOOKERS-ON noun- A spectator, onlooker.