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Three
of
four
proving
cryptic,
observed
puzzler?
(3)
Three of four proving cryptic, observed puzzler? (3)
hints
0
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The definition is at the end of the clue.
How the clue works
The definition is observed puzzler?, since a UFO is a sighted mystery. In the wordplay, "three of four" gives FOU (the first three letters of FOUR), and "proving cryptic" signals that these letters are rearranged to make UFO.