The most common and most likely answer to this crossword clue is the 12 letter word EYEWITNESSES
I've seen this crossword clue in The Times
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More clues leading to the result EYEWITNESSES
Buddy explains!
The straight part
We know that "They may when it does" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they may when it does = eyewitnesses.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 EYEWITNESSES mean?
EYEWITNESSES noun- Someone who sees an event and can report or testify about it.